Even though Democrats have long led on the generic ballot — they have a nearly 8 point advantage, on average — Biden is not just ahead because voters prefer Democrats. He’s ahead because voters support Biden more than they support the average Democrat running for Congress.1
In fact, when asked if they have a favorable or unfavorable attitude toward Biden, voters have either remained positive or split. That is a big contrast with 2016, when voters had consistently negative attitudes toward Clinton (and Trump).
One possible reason Biden might have a popularity edge? General election voters still perceive Biden as relatively moderate, and historically, more moderate presidential candidates generally do better in general elections. They generate less opposition when it comes to voter turnout and are perceived as less extreme by swing voters — though that advantage is declining. In fact, Trump benefited from this dynamic in 2016, when he was perceived as more moderate than most Republican candidates. (Although when we compare the current cycle to 2016, there is the separate question of gender, which is difficult to entangle but shouldn’t be discounted as a possible factor.)
Further testament to Biden’s strength as a general election candidate, though, is the fact that he continues to do better than Democratic primary runner-up Sen. Bernie Sanders in head-to-head matchups against Trump.