At first blush, Thursday’s release of census data held great news for Democrats. It painted a portrait of a considerably more urban and metropolitan nation, with increasingly Democratic metropolitan areas bustling with new arrivals and the rural, Republican heartland steadily losing residents.

It is a much less white nation, too, with the white non-Hispanic population for the first time dropping in absolute numbers, a plunge that exceeded most experts’ estimates, and the growth in the Latino population slightly exceeding forecasts.

But the census paints a picture of America as it is. And as it is, America is not very Democratic.

Besides the census, the other great source of data on American politics is the result of the 2020 election, which revealed a deeply and narrowly divided nation. Despite nearly the full decade of demographic shifts shown by the census, Joe Biden won the national vote by the same four-point margin that he won by as Barack Obama’s running mate eight years earlier — and with fewer votes in the Electoral College.

Democrats face great challenges in translating favorable demographic trends into electoral success, and the new census data may prove to be only the latest example. While the census shows that Democratic-leaning groups represent a growing share of the population, much of the population growth occurred in the Sun Belt, where Republicans still control the redistricting process. That gives them yet another chance to preserve their political power in the face of unfavorable demographic trends. And they are well prepared to do so.

The new data will be used by state legislatures and commissions to redraw electoral maps, with the potential to determine control of Congress and state legislatures across the country in next year’s midterm election.

Thursday’s release, the most detailed yet from the 2020 census, depicted a nation that increasingly seems to resemble its future more than its past. The non-Hispanic white share of the population fell to 57.8 percentage points, nearly two points lower than expected, as more Americans identified as multiracial. Vast swaths of the rural United States, including an outright majority of its counties, saw their populations shrink.

“Democrats have reason to be happy with this census data set,” said Dave Wasserman, House editor of the Cook Political Report, who cited the higher-than-expected population tallies in New York and Chicago and the steady growth of the nation’s Hispanic population.

Many Democrats had feared that Latino and urban voters would be badly undercounted amid the coronavirus pandemic and the Trump administration’s effort to ask about citizenship status.

It is still possible that the census undercounted Hispanics, but the results did not leave any obvious evidence that the count had gone awry. The Hispanic share of the population was in line with projections. New York City, the epicenter of the pandemic, showed unexpectedly strong population growth.

The surprising decline in the white and rural population is likely to bolster Democratic hopes that demographic shifts might help progressives secure a significant electoral advantage.

But the possibility that demographic changes would doom conservatives has loomed over American politics for more than a decade, helping to exacerbate conservative fears of immigration and even to motivate a wave of new laws intended to restrict access to voting. Tucker Carlson, the Fox News television show host, has repeatedly stoked racist fears of “white replacement,” warning his viewers that it is a Democratic electoral strategy.

Yet despite the seemingly favorable demographic portrait for Democrats depicted by the 2020 census, the 2020 election returned another closely divided result: a 50-50 Senate, one of the closest presidential elections in history, and a House majority so slender that it might be undone by the very data that Democrats were celebrating on Thursday.

The nation’s electoral system — which rewards flipping states and districts — has tended to mute the effect of demographic change. Many Democratic gains in vote margins have come in metropolitan areas, where Democratic candidates were already winning races, or in red states like Texas, where Democrats have made huge gains in presidential elections but haven’t yet won many additional electoral votes.

But Democrats haven’t fared much better over the past decade, as one would have expected based on favorable demographic trends alone. It’s not clear they’ve improved at all. Barack Obama and Joe Biden each won the national popular vote by four percentage points in 2012 and 2020. Demographic shifts, thus far, have been canceled out by Republican gains among nonwhite and especially Latino voters, who supported Mr. Trump in unexpectedly large numbers in 2020 and helped deny Democrats victory in Florida.

The new census data confirms that the nation’s political center of gravity continues to shift to the Republican Sun Belt, where demographic shifts have helped Democrats make huge inroads over the past decade. Georgia and Arizona turned blue in 2020. Texas, where Hispanic residents now roughly equal non-Hispanic whites, is on the cusp of becoming a true battleground state.

Just 50.1 percent of Georgians were non-Hispanic whites, according to the new census data, raising the possibility that whites already represent a minority of the state’s population by now.

But despite Democratic gains in the Sun Belt, Republicans continue to control the redistricting process in most of the fast-growing states that picked up seats through reapportionment.

The relatively robust number of Latino and metropolitan voters will make it more difficult for Republicans to redraw some maps to their advantage, by requiring them to draw more voters from rural Republican areas to dilute urban and metropolitan concentrations of Democratic-leaning voters. It may also help Democrats redraw maps to their favor in Illinois and New York, where they do control the redistricting process.

But there are few limits on gerrymandering, and even today’s relatively favorable data for Democrats are unlikely to be enough to overcome the expected Republican advantages in states where they enjoy full control over the redistricting process.

The Democrats may be relying on the Republicans’ growing bashful about gerrymandering, said Michael McDonald, a political science professor at the University of Florida.

“What the Republicans will have to do is crack the urban areas, and do it pretty aggressively,” he said. “It’s just one of those things we’ll have to see — how aggressive Republicans can be.”

Nick Corasaniti contributed reporting.