The erosion of support for Mr. Biden has been especially steep among independents, for reasons that cut to the heart of his presidency. During his campaign, he sent two basic messages, one to his party, the other to his country. He promised to bring Democrats together around an agenda carefully negotiated before the 2020 election began, as the leader of a party in which all Democrats from the center to the left would have a voice. At the same time, he would bring Americans back together by treating Republicans with respect and by doing his best to craft policies that appealed to both parties.

In practice, these two promises have proved incompatible. There have been some discrete bipartisan successes, such as the infrastructure bill and a measure to boost investment in technologies to counter China. But there is no Republican support for Democratic approaches to social programs, voting rights, immigration, criminal justice and public education.

Faced with a choice between party unity and national unity, Mr. Biden has chosen the former more consistently than independents had expected, and their disappointment is showing up in the polls. He will have a hard time regaining their support without trying harder to reach across party lines. But in today’s polarized climate, such a démarche might well fail.