In the run-up to the 2020 election, more voters across the country identified as Democrats than Republicans. But four years into Joseph R. Biden Jr.’s presidency, that gap has shrunk, and the United States now sits almost evenly divided between Democrats and Republicans.

Republicans have made significant gains among voters without a college degree, rural voters and white evangelical voters, according to a new report from the Pew Research Center. At the same time, Democrats have held onto key constituencies, such as Black voters and younger voters, and have gained ground with college-educated voters.

The report offers a window into how partisan identification — that is, the party that voters tell pollsters they identify with or lean toward — has shifted over the past three decades. The report groups independents, who tend to behave like partisans even if they eschew the label, with the party they lean toward.

“The Democratic and Republican parties have always been very different demographically, but now they are more different than ever,” said Carroll Doherty, the director of political research at Pew.

The implications of the trend, which has also shown up in party registration data among newly registered voters, remains uncertain, as a voter’s party affiliation does not always predict who he or she will select in an election. But partisan affiliation patterns do offer clues to help understand how the shifting coalitions over the last quarter century have shaped recent political outcomes. During the Trump administration, the Democratic Party’s coalition grew, helping to bring about huge victories in the 2018 midterm elections and a victory for President Biden in 2020.

The G.O.P. has long struggled with the fact that there have generally been fewer Americans who identified as Republicans than as Democrats. After Barack Obama was re-elected as president in 2012, the Republican Party produced an autopsy report that concluded that in order to be successful in future elections, the party would need to widen its tent to include Black and Hispanic voters, who were not traditionally aligned with the G.O.P.

Twelve years later, the party has made some small gains with Hispanic voters. But it is growth with the white working class and with rural voters that has propelled Republicans to equity with Democrats.

The catch is that white working-class voters are slowly declining as a share of registered voters, so the Republican strategy of relying heavily on the group may not be a sustainable in the long term.

At the same time, a much talked-about broad political realignment among Black and Hispanic voters has yet to materialize, at least by the metric of party identification.

Republicans’ growing strength with white working-class voters represents one of the biggest political schisms in the country over the past 15 years. In the 1990s and early 2000s, Democrats had a slight partisan identification advantage among voters without a college degree, while college-educated voters were more evenly divided between the two parties. Beginning in the early 2010s — and accelerating during the presidency of Donald J. Trump — voters without a college degree, in particular white voters without a degree, increasingly moved toward the Republican Party.

Now, nearly two-thirds of all white noncollege voters identify as Republicans or lean toward the Republican Party.

And Republicans are making gains among white women, as well. In 2018, a year after the Women’s March that attracted millions to protest Mr. Trump’s policies, the group was split about evenly between Democrats and Republicans. But since then, Republicans have slowly been gaining ground. They now hold a 10 percentage point partisanship advantage.

Overall, over most of the last 30 years, white voters have been more likely to identify as Republicans than Democrats, though the gap closed briefly in the mid-2000s.

While Hispanic voters are still far more likely to identify as Democrats, the party’s edge with the group has narrowed in the past few years. Currently, 61 percent of Hispanic voters identify as Democrats or lean toward the Democratic Party, down from nearly 70 percent in 2016. That trend mirrors polling in 2020 and 2024 that has shown the potential for support for Mr. Trump to grow among Hispanic voters.

That change appears most notable among Hispanic voters who do not have a college degree or who identify as protestant. As recently as 2017, the latter group leaned Democratic; now, it is more likely to identify as Republican, even as Hispanic Catholics are still more likely to identify as Democrats.

These shifts in partisanship fall short of what some predicted to be a political realignment, said Bernard Fraga, an associate professor of political science at Emory University who studies Latino voters.

But Latino voters care deeply about the economy, Mr. Fraga noted, and Latinos who are ideologically conservative are interested in Republicans and their plans for the country.

“It is also important to remember that the Latino population is extremely dynamic,” Mr. Fraga said. “There are a tremendous number of newly eligible voters in every election cycle. And what we perceive as a change or shift for Latinos is going to be disproportionately due to new voters.”

“About one third of Latino voters weren’t even in the electorate before 2016,” he added.

Black voters still overwhelmingly associate with the Democratic Party: Eighty-three percent of Black voters identify as Democrats or lean Democratic. There has been a small decline since 2020, when the share of Black voters who identified as Democrats was about five percentage points higher. Among Black men, 15 percent currently identify as Republicans, the same share as 30 years ago.