Progressives are tying themselves in knots to avoid giving President Trump credit for anything positive. Take the economy. It isn’t really booming, they say, but even if it is, President Obama is the man to thank.

The claim doesn’t add up. In 2010 the Obama White House forecast gross domestic product growth would “accelerate in 2011 to 3.8%” and “exceed 4% per year in 2012-2014,” consistent with the 4.3% growth rate in the other 10 recoveries since World War II. That never happened. Actual post-recession growth averaged an...