Why has this year’s presidential race been such an unnerving spectacle? It has been bitterly partisan and rhetorically unhinged—but so was the 1800 contest between Thomas Jefferson and John Adams. It has been framed by war, political violence and a sudden withdrawal by the incumbent—as was 1968. Donald Trump is a convention-defying roughneck who claims the last election was stolen from him—reprising Andrew Jackson in 1828. Kamala Harris was nominated by party insiders without consulting the voters—as was the rule before the mid-20th century.

Yet these comparisons fail to capture the singular upheavals of our times. The collapse of common culture. Moral confusion and ignorance among supposedly educated people. Pervasive corruption and dishonesty in public life. The infestation of the woke mind virus throughout institutions of education, media, culture and commerce.

I have a more specific explanation to add to the mix. The presidency has become much more powerful and less benign than it was designed to be. In recent years, the president has become lawmaker in chief, eclipsing Congress in many arenas of national life. He employs his new powers unabashedly for partisan purposes. His traditional, irreplaceable function as head of state and national leader has fallen by the wayside.

In many areas, such as tax and foreign policy, today’s president is, as ever, both uniquely important and politically constrained—he pursues his electoral mandate by collaborating with Congress and contending with international exigencies. But in many other areas he makes policy on his own. So that this year’s election, in determining who will be president for four years, will also determine whether to leave the southern border open, whether to restructure the power industry and phase out gas stoves and internal-combustion automobiles, and whether to require schools to let boys who feel they are girls compete in girls’ sports.

On these issues, the candidates’ policies are clear and opposite and won’t require further political assent. We know very well what Mr. Trump will do if elected. The same is true of Ms. Harris, despite her electioneering feints. I have mentioned a few high-profile issues. There are innumerable others in the executive tool kit, many of them strictly transactional and familiar only to specialists.

Little wonder that presidential campaigns have become billion-dollar enterprises, devoted mainly to partisan and interest-group mobilization rather than courting the political center. It is telling that both sides have said democracy itself is at stake—for choosing the president is now what our democracy mainly consists of. Both sides employ highly precise data and communications technologies that guarantee the final result will turn on a few votes in a few states. That so much is at stake in an election decided by a sliver of the electorate, perhaps involving weeks of uncertainty and legal challenges, is a sure recipe for popular anxiety and cynicism.

We have arrived at these straits through 50 years of gradual political evolution, as one incremental step led to another with little appreciation of where we were going.

In the 1970s, Congress created numerous executive-branch agencies to manage newly salient political issues—the Environmental Protection Agency, the National Highway Traffic Safety Administration, the Occupational Safety and Health Administration, the Energy Department, the Education Department, and many civil-rights offices to adjudicate claims of discrimination.

These new offices possessed unprecedented power to issue sweeping national rules, often costing hundreds of millions of dollars apiece. Presidents noticed that they were getting the praise and blame for policies on which they weren’t in the loop. They began to require agencies to submit draft rules to the White House and Office of Management and Budget, leading to President Reagan’s requirement that OMB review all rulemaking proposals under a cost-benefit standard.

In the 1970s and ’80s, the review programs aimed to make regulations more cost-effective and attuned to economic incentives. But the process familiarized presidents and White House staffers with the extraordinary leeway agencies possessed in interpreting their statutory mandates. It eventually occurred to them to use the statutes in pursuit of the president’s own priorities.

Presidential unilateralism began in 1995, when Bill Clinton, with great fanfare, directed the Food and Drug Administration to regulate tobacco products. Then, during the 2008 financial crisis, George W. Bush’s administration arranged shotgun mergers of weak firms into stronger ones and bailed out Chrysler and General Motors. The legal justifications for these moves were far-fetched, beyond what the most zealous agency would have attempted on its own.

The Clinton rule was a personal crusade and the Bush actions an emergency response. But they paved the way for Barack Obama’s “pen and phone” initiatives, which were both unilateral and partisan. His Clean Power Plan and “deferred action” immigration measures were explicit responses to Congress’s refusal to pass his legislative proposals. They were justified by legal legerdemain and were politically one-sided, omitting provisions Republicans would have insisted on in a legislative compromise. His “guidance” on transgender use of school bathrooms was partisan in a different way—it satisfied a narrow party constituency on an unpopular policy that congressional Democrats didn’t want to vote on.

Donald Trump was a strong deregulator, and he often berated Congress but seldom defied it. Indeed, he proposed to support some of Mr. Obama’s unilateral immigration and health-care policies if Congress enacted them along with policies of his own. But even he resorted to unilateralism in banning “bump stocks” that allow semiautomatic rifles to fire as fast as fully automatic ones—a popular step following the 2017 Las Vegas bump-stock massacre that congressional Republicans preferred not to vote on.

President Biden has taken extra-statutory lawmaking to extremes that would have been unthinkable in earlier administrations—canceling $430 billion in student loans, ordering Covid vaccinations throughout the private sector, and extending a Covid-lockdown legislative ban on apartment evictions. The point of his “all of government” initiatives—to promote “diversity, equity and inclusion” and to suppress fossil fuels—is to commandeer agency mandates for his own purposes. They put the Securities and Exchange Commission and Federal Reserve Board on the climate-change beat, and charge the FDA and National Weather Service with promoting “equity” for favored identity groups.

Presidential lawmaking hasn’t been a seizure of power like a military coup. It might be described as the opposite—legislative abdication. When I was working for President Richard Nixon on environmental policy in 1970, he asked Congress to enact a national tax on sulfur-dioxide emissions. We couldn’t find a single lawmaker of either party to introduce the bill we had helpfully drafted. The congressional preference was to vote for clean air in the abstract and leave the hard, costly choices to the executive.

But we might, just this once, work up a little sympathy for Congress. For 60 years it has been inundated with “affluent society” issues generated by an educated, networked, comfort-seeking electorate. They have overwhelmed the capacities of a committee-laden, conflict-riven legislature. It was a natural response to delegate the issues to missionary executive agencies, where policy conflicts are more specialized and decision-making is relatively streamlined. This practice has endured: Mr. Obama’s two big legislative successes, ObamaCare and Dodd-Frank, empowered fleets of executive bureaus and committees with broad lawmaking powers.

In sum, modern society politicized many hitherto private matters and centralized much of government in Washington, which led Congress to delegate much of its new business to executive agencies, which led the one elected executive to take charge of the vast bureaucratic empire.

The result has been a severe erosion of the rule of law. Legislative law is forged by give and take among representatives of disparate regions, interests and values and typically embodies a broad consensus. Citizens who dislike a result can take patriotic solace that their voices were part of the deliberations and they may have scored a few compromises. Executive law lacks this social depth. Although the president has won a majority of the Electoral College and usually a popular majority or plurality, he resorts to personal lawmaking precisely to evade Congress’s representative gauntlet, and often to serve partisan interests. This form of law accentuates rather than mediates our differences.

Moreover, executive law is unstable and undependable. For all its appearance of decisive leadership, it has a fleeting, mercurial character. Changes in administration from one party to the other bring extensive revisions to federal law, including outright reversals, as in the fervent back-and-forth rewriting of environmental, financial, immigration, charter-school, civil-rights and sexual-relations laws under Presidents Obama, Trump and Biden. Legal instability discourages civic compliance, economic investment and forward thinking of all sorts. Above all, it incites public cynicism about the subservience of law to political winds and executive will.

Can the courts preserve a modicum of the rule of law? The Supreme Court eventually rejected the Clinton smoking rule, the Obama Clean Power Plan, the Trump bump-stock ban and the Biden vaccine mandate and other forays. The court is tightening up its lackadaisical doctrines that have encouraged Congress to delegate its lawmaking powers and the executive to improvise freely.

But the fusion of lawmaking and law enforcement has produced a dynamic that can outmaneuver judicial process. When the justices rejected Mr. Biden’s student-loan cancellation, he had Plan B ready to go. The courts have paused one of its elements, but others have moved forward, amounting to $175 billion in cancellations, more are in the works, and, let’s face it, not collecting debts is easy. The Obama and Biden immigration policies have relied heavily on mass nonenforcement of selected statutory provisions—which puts courts in a bind, because policing and prosecutorial discretion are core executive functions on which courts are rightly loath to intrude and poorly suited to exercise.

The most striking example is Mr. Biden’s extension of the Covid eviction moratorium. He said he had been advised that the action was unconstitutional—but at least he could keep delinquent renters in their homes until the courts so ruled, which is exactly what happened. It was a brazen violation of his oath of office.

And the executive now uses enforcement discretion to corral large corporations to its policies in ways that are hard for courts to control. Collaborative debanking and social-media censorship of political opponents sometimes come to public notice, but regulatory enforcement in one area or noncooperation in another area is essentially impossible to police.

Restoring the presidency to its proper role will likely require leadership from the president himself. This will sound quixotic. Presidents are power-hungry politicians—that’s how they got there—and they have come to relish the role of lawgiver. Messrs. Obama, Trump, and Biden have staged photogenic ceremonies for signing executive orders and even routine memorandums. The events resemble legislative signing ceremonies where presidents are surrounded by representatives and senators who shepherded the bill to passage. But in the new ceremonies, presidents are surrounded by their own staffs and appointees, who applaud his signing a document that they wrote, addressed to themselves.

Yet the presidency has become a sorcerer’s stone, an enchanted prize that curses its holder. You are expected to enact policies that your party doesn’t want to vote for and that the next president may throw out if the courts don’t get there first. You totally own an invasive, divisive, distrusted government, and you personify the era’s rancorous politics. You are no longer the national leader the president was expected to be. Every president since Bill Clinton has been subjected to continuous, well-organized assaults on his character and legitimacy.

At some point, presidents are going to realize that sharing responsibility with Congress for major policy departures is good not only for the nation but for their own tenure and legacy. In his 2012 presidential campaign, Mitt Romney vowed that as president he would refer major initiatives to Congress for approval, whether or not they were authorized by statutes on the books. Mr. Trump’s White House staff prepared a similar initiative, but held back when his relations with Speaker Nancy Pelosi deteriorated. Whichever party’s president makes the first move, Congress will resist mightily. But the ruckus would be edifying, and could move our politics and our presidency toward a better place, the sturdy place of representative law.