But a bigger shadow lurks over the Democratic field: the ghost of the Republican presidential campaign of 2016, when the candidates (like Jeb Bush) who attacked the outsider with the intense fan base lived to regret it. If you attack Sanders, and his democratic socialist platform, as mathematically challenged, you are not just attacking Sanders. You are attacking democratic socialism itself. And if you’re in a party with a young wave of democratic socialists as its newest and most unpredictable force, you risk disaster.

No one can say with certainty how many Sanders supporters would abandon the Democratic nominee if he lost the nomination. But we do know that his supporters are, on average, less loyal to the Democratic Party than voters who prefer other candidates. The Economist’s data guru G. Elliott Morris reported, based on two months of his operation’s polling toward the end of last year, that 87 percent of Sanders supporters would stick with the Democrats if he wasn’t the nominee. That’s a lot, but more than 90 percent of Biden, Buttigieg, Klobuchar and Warren supporters said they would vote for the Democrats this fall, no matter what. And just a few percentage points, if even that, could decide the presidency.

When Biden, Buttigieg and Klobuchar go after Warren, they suspect they’re hurting a rival without costing themselves a potential vote in November. With Sanders, the math isn’t as simple. Estimates of the number of Sanders 2016 primary voters who cast a ballot for Donald Trump range from 6 to 12 percent. That’s actually fewer than the estimates of Hillary Clinton ’08 primary voters who backed Republican nominee John McCain over Barack Obama. But unlike 2008, the Sanders defectors may have been enough to tip Pennsylvania, Michigan, Wisconsin and the presidency into the Republican column.