America Is Undergoing Seismic Changes. Its Politics? Hardly.

In another age, the events of this season would have been nearly certain to produce a major shift in American politics — or at least a meaningful, discernible one.

Over a period of weeks, the coronavirus death rate plunged and the country considerably eased public health restrictions. President Biden announced a bipartisan deal late last month to spend hundreds of billions of dollars rebuilding the country’s worn infrastructure — the most significant aisle-crossing legislative agreement in a generation, if it holds together. The Congressional Budget Office estimated on Thursday that the economy was on track to regain all of the jobs it lost during the pandemic by the middle of 2022.

And in a blow to Mr. Biden’s fractious opposition, Donald J. Trump — the dominant figure in Republican politics — faced an embarrassing legal setback just as he was resuming a schedule of campaign-style events. The Manhattan district attorney’s office charged his company, the Trump Organization, and its chief financial officer with “sweeping and audacious” financial crimes.

Not long ago, such a sequence of developments might have tested the partisan boundaries of American politics, startling voters into reconsidering their assumptions about the current president, his predecessor, the two major parties and what government can do for the American people.

These days, it is hard to imagine that such a political turning point is at hand.

“I think we’re open to small moves; I’m not sure we’re open to big moves,” said Mark Mellman, a Democratic pollster. “Partisanship has made our system so sclerotic that it isn’t very responsive to real changes in the real world.”

Amid the mounting drama of the early summer, a moment of truth appears imminent. It is one that will reveal whether the American electorate is still capable of large-scale shifts in opinion, or whether the country is essentially locked into a schism for the foreseeable future, with roughly 53 percent of Americans on one side and 47 percent on the other.

Mr. Biden’s job approval has been steady in the mid-50s for most of the year, as his administration has pushed a shots-and-checks message about beating the virus and reviving the economy. His numbers are weaker on subjects like immigration and crime; Republicans have focused their criticism on those areas accordingly.

This weekend, the president and his allies have mounted something of a celebratory tour for the Fourth of July: Mr. Biden headed to Michigan, one of the vital swing states that made him president, while Vice President Kamala Harris traveled to Las Vegas to mark a revival of the nation’s communal life.

On Friday, Mr. Biden stopped just short of declaring that happy days are here again, but he eagerly brandished the latest employment report showing that the economy added 850,000 jobs in June.

“The last time the economy grew at this rate was in 1984, and Ronald Reagan was telling us it’s morning in America,” Mr. Biden said. “Well, it’s getting close to afternoon here. The sun is coming out.”

Yet there is little confidence in either party that voters are about to swing behind Mr. Biden and his allies en masse, no matter how many events appear to align in his favor.

Democratic strategists see that as no fault of Mr. Biden’s, but merely the frustrating reality of political competition these days: The president — any president — might be able to chip away at voters’ skepticism of his party or their cynicism about Washington, but he cannot engineer a broad realignment in the public mood.

Mr. Mellman said the country’s political divide currently favored Mr. Biden and his party, with a small but stable majority of voters positively disposed toward the president. But even significant governing achievements — containing the coronavirus, passing a major infrastructure bill — may yield only minute adjustments in the electorate, he said.

“Getting a bipartisan bill passed, in the past, would have been a game changer,” Mr. Mellman said. “Will it be in this environment? I have my doubts.”

Russ Schriefer, a Republican strategist, offered an even blunter assessment of the chances for real movement in the electorate. He said that the receding of the pandemic had helped voters feel better about the direction the country is moving in — “the Covid reopening certainly helps with the right-track numbers” — but that he saw no evidence that it was changing the way they thought about their preferences between the parties.

American voters’ stubborn resistance to external events is no great surprise, of course, to anyone who lived through the 2020 election. Last year, Mr. Trump presided over an out-of-control pandemic that killed hundreds of thousands of people and caused the American economy to collapse. He humiliated the nation’s top public health officials and ridiculed basic safety measures like mask wearing; threatened to crush mass demonstrations with military force; outlined no agenda for his second term; and delivered one of the most self-destructive debate performances of any presidential candidate in modern history.

Mr. Trump still won 47 percent of the vote and carried 25 states. The trench lines of identity-based grievance he spent five years digging and deepening — pitting rural voters against urban ones, working-class voters against voters with college degrees, white voters against everybody else — saved him from an overwhelming repudiation.

A Pew Research Center study of the 2020 election results released this past week showed exactly what scale of voter movement is possible in the political climate of the Trump era and its immediate aftermath.

The electorate is not entirely frozen, but each little shift in one party’s favor seems offset by another small one in the opposite direction. Mr. Trump improved his performance with women and Hispanic voters compared with the 2016 election, while Mr. Biden expanded his party’s support among moderate constituencies like male voters and military veterans.

The forces that made Mr. Trump a resilient foe in 2020 may now shield him from the kind of exile that might normally be inflicted on a toppled former president enveloped in criminal investigations and facing the prospect of financial ruin. Polls show that Mr. Trump has persuaded most of his party’s base to believe a catalog of outlandish lies about the 2020 election; encouraging his admirers to ignore his legal problems is an old trick by comparison.

The divisions Mr. Trump carved into the electoral map are still apparent in other ways, too: Even as the country reopens and approaches the point of declaring victory over the coronavirus, the states lagging furthest behind in their vaccination campaigns are nearly all strongholds of the G.O.P. While Mr. Trump has encouraged his supporters to get vaccinated, his contempt for public health authorities and the culture of vaccine skepticism in the right-wing media has hindered easy progress.

Yet the social fissures that have made Mr. Trump such a durable figure have also cemented Mr. Biden as the head of a majority coalition with broad dominance of the country’s most populous areas. The Democrats do not have an overwhelming electoral majority — and certainly not a majority that can count on overcoming congressional gerrymandering, the red-state bias of the Senate and the traditional advantage for the opposition party in midterm elections — but they have a majority all the same.

And if Mr. Biden’s approach up to this point has been good enough to keep roughly 53 percent of the country solidly with him, it might not take a major political breakthrough — let alone a season of them — to reinforce that coalition by winning over just a small slice of doubters or critics. There are strategists in Mr. Biden’s coalition who hope to do considerably more than that, either by maneuvering the Democratic Party more decisively toward the political center or by competing more assertively with Republicans on themes of economic populism (or perhaps through some combination of the two).

Mr. Biden’s aides have already briefed congressional Democrats several times on their plans to lean hard into promoting the economic recovery as the governing party’s signature achievement — one they hope to reinforce further with a victory on infrastructure.

Faiz Shakir, who managed Senator Bernie Sanders’s 2020 presidential campaign, said Democrats did not need to worry about making deep inroads into Mr. Trump’s base. But if Mr. Biden and his party managed to reclaim a sliver of the working-class community that had recently shifted right, he said, it would make them markedly stronger for 2022 and beyond.

“All you need to focus on is a 5 percent strategy,” Mr. Shakir said. “What 5 percent of this base do you think you can attract back?”

But Mr. Shakir warned that Democrats should not underestimate the passion that Mr. Trump’s party would bring to that fight, or the endurance of the fault lines that he had used to reorganize American politics.

“He has animated people around those social and racial, cultural, cleavages,” Mr. Shakir said of Mr. Trump. “That keeps people enthused. It’s sad but it is the case that that is going on.”