We know what happened next. Instead of focusing on economic growth, the Obama administration advanced an expensive but ineffective stimulus package and then went all in on a partisan transformation of the U.S. health-care system. The upshot was the slowest economic recovery since World War II. Democrats lost their House majority in 2010, and Republicans won the Senate four years later.

We’re about to find out what, if anything, Mr. Biden and his more liberal allies took away from that experience. Mr. Biden has said that his first order of business as president will be addressing Covid-19 and the economy. What we don’t know is whether the political left intends to let him do that. The progressive wing of the Democratic Party is much stronger than it was in 2009, and its members of Congress are far more numerous. Back then, Bernie Sanders and Elizabeth Warren did not command the national attention that they do today, and Alexandria Ocasio-Cortez wasn’t old enough to buy beer.