Once upon a time, American liberals loved to hate foreign-policy realists. Richard Nixon and Henry Kissinger - the uber-realists of their day - were the betes noires of the left. In the liberal view, stability and Realpolitik were the source of everything wrong with U.S. foreign policy.

No more. In an ideological shift that should make Mr. Nixon turn over in his grave, liberal internationalism is making peace with its erstwhile intellectual enemy, the tradition of realism in U.S. foreign policy. Liberals and realists are joining hands to forge a new vision of American leadership that President Obama may be tempted to embrace.

Liberal internationalism is a familiar school of thought. In the tradition of Woodrow Wilson and Franklin D. Roosevelt, its advocates called for a strong American leadership role to transform the world order. But since the Vietnam War, the emphasis has been less on transforming the world and more on establishing an international consensus that accommodates the world's interests.

American realism used to be liberal internationalism's polar opposite. It looked askance at liberalism's messianic impulse to change the world, fearing that could lead to a dangerous overextension of U.S. power. But the presidency of George W. Bush changed that. The old liberal ideological impulse to change the world is now supposedly the property of "neo-conservatives" - those Cold War liberals who migrated to conservatism during Mr. Reagan's presidency. Traditional realists like Brent Scowcroft and Alexander Haig, otherwise known as conservatives, now saw the dreaded excessive moralistic impulse in Mr. Bush's Iraq war, not in the liberals who opposed it.

Now realism is all the rage, even in liberal circles. New Mexico Gov. Bill Richardson criticizes Mr. Bush for his "lack of realism" in flouting international consensus. Brookings Vice President Carlos Pasqual praises the new Obama national security team as representing a "new realism."

It's as if liberals and realists traded places. Some liberals seem to have morphed from starry-eyed idealists into hard-nosed realists. While many liberals worry deeply about genocide in Darfur, others like Mr. Obama are cool to the idea of using military force to solve the problem. Liberals who complain incessantly about human rights abuses in China and Russia find themselves opposing hard-line policies against these countries, fearing they may lead to confrontation and disturb the international consensus.

Why have so many liberals rushed to embrace realism? A major reason is liberal furor over the Iraq war. Having turned against the perceived ideological impulse behind that war (namely, the spread of freedom and democracy through military means), they now also doubt the traditional liberal mission of using American power to transform the world. Today many liberals believe more in accepting the world as it is than in changing it.

And here they are in perfect harmony with the realists. New realists like Newsweek International editor Fareed Zakaria embrace the liberal penchant for international consensus, believing it will keep America from overextending itself. A traditional realist like Mr. Scowcroft is encouraged by Mr. Obama because he hopes "that, at least for a time, America has had enough of transforming the world." Realists in the past may have doubted the central need for the United Nations and international consensus, but today they see them as indispensable means to restrain America from excessive moralizing.

There is a hint of this liberal-realist fusionism in Mr. Obama, but the jury is still out on whether he will embrace it completely. The realist critique of Mr. Bush and the Iraq war fits like a glove on Mr. Obama's view that America needs to adjust to the realities of the new "multipolar" world. And Mr. Obama is willing to shed liberalism's excessive idealism if it endangers international consensus. In the case of Darfur, his liberal desire for consensus - the realists' new form of "stability" - trumps the old liberal idea of transforming the world.

This convergence of worldviews extends beyond Big Power politics to the perennial debate over which is more important - America's values or interests. Mr. Obama and the realists believe America must accommodate the rise of Chinese power and avoid starting another Cold War with Russia, regardless of concerns over human rights. Their interest in accommodation aligns nicely with the desire to preserve international consensus and thus prevent an overextension of America's reach.

Many liberals and realists seem to believe that America is declining as a world power. Their reactions to this idea range from gleeful to mournful; but the preoccupation with putting limits on America's ambitions raises questions about how durable they think America's staying power really is. Realists such as Robert Kaplan see a need for assertive American leadership, but even he is resigned to the "long and elegant decline" of the United States.

Mr. Obama embraces many elements of liberal-realist ideology, but as a politician he appears to think that it's not a good idea to give up on America. As he said in 2008, "To see American power in terminal decline is to ignore America's great promise and historic purpose in the world."

Mr. Obama is right. He seems to sense a need to impose some limits on the inherent pessimism of the new liberal-realist fusion. A vision of America riding off into the sunset of geopolitical decline does not square with his message of hope and change. Americans may not want the U.S. to be the world's policeman, but they also still believe their country has a transformative role to play in the world.

Most hope for America to play a role somewhat larger than simply honorary chairman of the board of international consensus. Besides, if the United States does not lead, who will? China, with its mixed command-market economy, one-party rule, disregard for international law, and apologiae for the genocides in Darfur? The United Nations, with its sad record of inefficiency and failure?

I don't think so. The world still needs American leadership. Not to guide the U.S. descent as a great power, and not to manage an international consensus reflecting the lowest common denominator of what 194 countries can agree upon. Rather, the world needs American leadership to help transform the world by pursuing its highest and best aspirations. It is fine to be realistic, but we should be wary when realism-induced "declinism" masquerades as prudence.