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,...