President Trump is negotiating with Congress over a massive stimulus plan to combat the severe economic and financial fallout from the coronavirus. One idea that seems to be catching on is a check of up to $1,200 to be mailed to every American, while Democrats in Congress want paid-leave policies and expanded welfare benefits. These may provide some needed temporary relief for families but are unlikely to help lift the economy. Keynesian stimulus almost always fails, and often makes the downturn worse and the eventual recovery weaker.

Mr....