Joe Biden heads into the Iowa caucuses, and thus actual voting in the Democratic presidential primaries, as the national front-runner. In other words, what he’s been for most of the campaign. According to the RealClearPolitics average, the former vice president holds a sizable polling lead in Nevada and South Carolina and a slim edge over socialist Bernie Sanders in Iowa and New Hampshire. Nationally, he leads the Democratic field by 8 points. Yet if there’s anything remarkable about Biden’s campaign, it’s how little enthusiasm anyone seems to have for it. Though his campaign stops are well attended, hundreds are not waiting for hours in line to get a photo with him as they are with Elizabeth Warren. He lacks ardent online advocates in the vein of Sanders’s “Bernie Bros” or Andrew Yang’s “Yang Gang.” On...