Karl Rove had a piece in The Wall Street Journal, and he had this comment about the situation in which Progressive-Democratic Party Presidential candidate Joe Biden finds himself:

Mr Biden must also decide soon whether to keep moving left or emphasize that he’s a more centrist [Progressive-]Democrat. Picking the first course would suggest that he believes victory this fall depends on mobilizing Senator Bernie Sanders’s [I,VT] backers by agreeing with many of the Vermonter’s views.
Picking the second would indicate that he thinks the key to victory lies with suburbanites who swung to Democrats in 2018, and that he’ll get the Bernie vote by being the alternative to Mr Trump, not the instrument to enact a socialist agenda.
But Mr Biden may already have moved too far left for some suburbanites….

Biden can’t tack back in the more-or-less general direction of the center (whether Party’s center, or America’s); to do so would utterly destroy his credibility. Both Party’s left and those suburbanites would see the move as blowing with the political-convenience winds inconstancy.

And so would the American electorate at large.

Biden is stuck.