Liberals are tripping over themselves to explain why the economy has performed so much better under Donald Trump than it did under Barack Obama. The economy has grown by nearly 4% over the past six months, and the final number for 2018 is expected to come in at between 3% and 3.5%. The U.S. growth rate has doubled since Mr. Obama’s last year in office.

When Mr. Trump was elected, many Democratic pundits predicted an economic and stock-market meltdown. Then the economy started surging and they abruptly changed their tune, arguing that Mr. Trump was simply riding a global growth wave. That narrative was shattered when U.S. growth kept steaming ahead even as global growth—especially in China and Germany—stalled.

The latest liberal spin is that the economy is on a “sugar high” from deficit-financed tax cuts and spending hikes. When the rush wears off, they warn, watch out for a crash landing. It’s true that in fiscal 2018 the budget deficit swelled to nearly $800 billion, or about 4.2% of gross domestic product. But the Bureau of Economic Analysis estimates that the economic “contribution” from extra government spending added only 0.23 percentage point to growth in 2018. So even without all the budget bloat, the economy would still be growing well above 3%.

The same BEA data confirm that this year’s growth comes predominantly from a boom in production and investing—particularly in construction, manufacturing, and oil and gas development. While the housing market is weak, consumers are spending more as their wages rise. Small-business and consumer confidence remain at or near historic highs.

The real contradiction in the “sugar high” argument is that it ignores the slow growth of the Obama years, which featured an avalanche of debt spending. Deficits as a share of GDP were 9.8% in 2009, 8.6% in 2010, 8.3% in 2011 and 6.7% in 2012. Where was the sugar high then? Instead of the expected burst in output coming out of the 2008-09 recession, borrowing more than $1 trillion a year for four years yielded the worst recovery since the Great Depression. Even excluding 2009, Mr. Obama’s deficits averaged more than 5% of GDP throughout the rest of his presidency but produced less growth than Mr. Trump has with lower deficits.

This wasn’t what Keynesians expected. Mr. Obama’s economic team predicted 4% growth every year coming out of the recession. Instead the “sugar high” from record peacetime deficits produced measly 2% growth. By 2016 GDP was running about $2 trillion below the trend line of a normal recovery.

The fastest growth rate over the past three decades was recorded in Bill Clinton’s second term, when federal government spending fell from 21.5% to 18% of GDP and deficits disappeared into surpluses. So much for the idea that deficit spending is a stimulant.

Mr. Trump’s fiscal policies have produced more growth than Mr. Obama’s because they were designed to incentivize businesses to invest, hire and produce more here at home. The Obama “stimulus,” by contrast, went for food stamps, unemployment benefits, ObamaCare subsidies, “cash for clunkers” and failed green energy handouts.

Massive government spending blitzes don't produce “sugar highs” or anything like them. Even some conservatives erroneously argue that military spending stimulates the economy. But as Milton Friedman said, the government can only put money into the economy that it first takes out.

Those pushing the “sugar high” fallacy also don’t realize that the Trump tax cuts aren’t going away soon. The 2017 business tax cuts can’t cause a recession in 2019 or 2020 because they don’t expire until 2025. They aren’t sugar pills.

The biggest threats to the economic boom and financial markets today are a deflationary Federal Reserve and the specter of a global trade war. Solve those problems and the American economy can keep flying high on its own power. And Mr. Trump’s critics will be proved wrong again.