Democrats Were Lukewarm on Campaign Biden. They Love President Biden.

But Mr. Biden may also be benefiting from some forms of progress that were not entirely of his own making. Millions of Americans are being vaccinated daily, moving the country closer to emerging from the coronavirus pandemic. As the United States moves slowly but steadily toward herd immunity, forecasters anticipate a quickly expanding economy, with even Republicans like Senator Mitch McConnell of Kentucky, the minority leader, predicting a financial boom that could last into 2023.

Mr. Biden took steps to hasten virus vaccine production, but some of his political success on that front can be attributed to savvy public positioning. By tamping down expectations for vaccine distribution during his first weeks in office, when Mr. Biden beat his own expectations, his team conjured an image of a White House working overtime to leave the efforts of the previous administration in the dust.

Though Mr. Trump laid the groundwork for widespread vaccine production with his Operation Warp Speed program, it is Mr. Biden who may be reaping the political benefit from that push — especially within his own party.

Indeed, Democrats’ antipathy for Mr. Trump has a lot to do with their fondness for the new president, said Whit Ayres, a veteran Republican pollster. “Democrats utterly detested Donald Trump and Joe Biden saved them from Donald Trump, and so they love him,” Mr. Ayres said. “If you look at the overall job approval, not just among Democrats, Biden’s job approval is the inverse of Donald Trump’s.”

Mr. Biden is hardly the first president to enjoy broad support from his party upon taking office. It is typical for commanders in chief to start their first term with a broadly positive approval rating, as Mr. Biden did, although that is always subject to the pull of gravity after the first few weeks are over.

But in the history of Gallup polling going back to the mid-20th century, Mr. Biden is the first president to have started his term with the approval of more than 90 percent of partisans.

To a degree, this reflects the fact that as the two major parties have grown more entrenched in their ideological identities, voters at the center have become slightly less likely to identify with either one. As a result, there has been a recent uptick in the share of Americans calling themselves political independents.