Biden, like Trump, believes in appealing to his base. That is a risky proposition in any event, since rallying your supporters may backfire if at the same time you rally everyone else to vote against you. Consider Hillary Clinton who denounced the “deplorables” – it encouraged them to turn out and elect Trump.
Biden has multiple risks. Most serious, Trump might not run, or might run and not be nominated (the recent Democratic victories in House races suggest that a Trump endorsement, while helping in a primary, undermines a candidate in an election – if neither Trump nor the rest of the Republicans want to support a “loser”). Polling suggests that most Americans are moderates, or as some might say, either independents, DINOs or RINOs – so while being a fanatic helps in the primaries it won’t help in the election.