For more than two weeks now, the Democratic Party has been mired in crisis. And yet there is no sign that the threat to Joe Biden’s reelection is nearing a conclusion. The president has dug in even as a growing chorus of Democratic officials, donors and strategists calls for him to step aside. Donors and high-profile endorsers are repudiating Biden, morale inside and outside the campaign is weak, and some top Democrats are pondering whether to make a move against the embattled president. One of Biden’s allies privately described a cycle of alternating hope and despair in the style of the movie “Groundhog Day.” Republicans, meanwhile, are celebrating the chaos heading into their convention that starts Monday.