For years, America’s congressional map favored Republicans over Democrats.

But that may not remain the case for long.

In a departure from a decades-long pattern in American politics, this year’s national congressional map is poised to be balanced between the two parties, with a nearly equal number of districts that are expected to lean Democratic and Republican for the first time in more than 50 years.

Despite the persistence of partisan gerrymandering, between 216 and 219 congressional districts, out of the 435 nationwide, appear likely to tilt toward the Democrats, according to a New York Times analysis based on recent presidential election results. An identical 216 to 219 districts appear likely to tilt toward Republicans, if the maps enacted so far withstand legal challenges. To reach a majority, a party needs to secure 218 districts.

The surprisingly fair map defies the expectations of many analysts, who had believed that the Republicans would use the redistricting process to build an overwhelming structural advantage in the House, as they did a decade ago.

As recently as a few months ago, it had seemed likely that Republicans could flip the six seats they needed to retake the House through redistricting alone. Instead, the number of Republican-tilting districts that voted for Donald J. Trump at a higher rate than the nation is poised to decline significantly, from 228 to a figure that could amount to fewer than the 218 seats needed for a majority.

Democrats could claim their first such advantage since the 1960s, when the Supreme Court’s “one person, one vote” ruling and the enactment of the Voting Rights Act inaugurated the modern era of redistricting.

The relatively fair map is something of an accident. Democrats and Republicans again drew extreme gerrymanders with twisting and turning district lines, denying many communities representation in Congress. Dozens of incumbents were shielded from serious challenges. The number of competitive districts declined.

But, unlike in previous cycles, both parties’ extreme gerrymanders have effectively canceled each other out — in no small part because Democratic lawmakers went to greater lengths to maximize their advantage. And more states are having maps drawn by courts or by nonpartisan and bipartisan commissions than in previous decades, reducing the number of districts drawn to intentionally advantage one party.

The final congressional map still remains uncertain, with Florida, Missouri, Louisiana and New Hampshire yet to finish drawing new lines and several more facing legal challenges. One party may ultimately wind up claiming several more districts than the other.

And the Republicans have a slight edge that’s easy to overlook: There are more solidly Republican than solidly Democratic districts, with 186 that voted Republican by at least 10 points more than the nation as a whole in the last presidential election, compared to 167 for the Democrats. The smaller number of solidly Democratic districts means that the party needs to win a somewhat larger share — perhaps 60 percent — of the potentially competitive districts to win a majority.

But the range of likely outcomes is narrowing, especially with several courts asserting that it is too late to address potentially unconstitutional maps before upcoming primary elections. On Monday, the Supreme Court denied Republican requests to block court-selected maps in Pennsylvania and North Carolina, cementing the Democratic hold on at least four districts where Republicans might have otherwise had an advantage. Last week, a New York State judge declined to block the state’s new Democratic gerrymander.

Even if the remaining four states enact the maps proposed by Republicans, the party would still only earn 218 total districts that tilt their way, compared to 217 for the Democrats.

The Times analysis looked at how a district tilted toward a party by examining whether it voted more for a party’s presidential candidate in the last election than the nation did as a whole. That metric is commonly used to judge the partisanship of congressional districts, since presidential and congressional election results are highly correlated.

The relatively fair congressional map reflects a decade-long effort by progressives to combat Republican-led gerrymanders in courts, legislatures and at the ballot box. While proposals to reform redistricting are nothing new, they took on new urgency for Democrats in the aftermath of the last redistricting cycle, when Republicans enacted aggressive gerrymanders that gave the party a considerable structural advantage. The party easily retained control of the House in the 2012 election, even while Democrats won the most votes.

At its peak in 2016, the Republican structural advantage was daunting. Just 195 districts tilted toward Hillary Clinton in that year’s presidential election, compared to 240 that tilted toward Mr. Trump. The median congressional district voted for Mr. Trump by nearly four percentage points, six points more favorable to the Republicans than Mr. Trump’s two-point deficit in the national popular vote. The outcome raised the possibility that Democrats could only win the House in a national landslide.

But the Republican advantage crumbled, even before this cycle’s redistricting began. A string of court rulings in North Carolina, Florida, Pennsylvania and Virginia eroded or eliminated some of the party’s most valuable gerrymanders, cutting the chamber’s Republican edge down by one-third before the 2020 election.

At the same time, unfavorable electoral trends eroded the Republican Party’s longstanding geographic advantage: the tendency for the party to more efficiently translate its votes into seats than the Democrats, who win lopsided margins in urban areas but tend to lag in less populous areas. Mr. Trump’s weakness in traditionally competitive suburbs — along with his relative strengths in less competitive rural and urban areas — made his coalition somewhat less effective at winning House seats than for prior Republicans. It cut the Republican advantage down by half.

Together, the diminishing Republican geographic advantage and weakened gerrymanders were just enough for Democrats to narrowly win the House with a modest win in the popular vote in 2020.

Republicans were expected to reclaim their advantage again this cycle, as the party would draw more seats than the Democrats. But Republicans had fewer opportunities to improve over their prior maps. In some states, new court rulings and constitutional amendments limited what Republicans could do with their powers. In others, Republicans had already drawn the lines so overwhelmingly to their advantage a decade earlier that there were few opportunities for them to go much further. They chose to reinforce more vulnerable incumbents as often as they eliminated additional Democratic seats.

Democrats, on the other hand, had more opportunities to be more aggressive than they had been a decade ago. Their victories in the 2018 midterm elections gave them more influence in the redistricting process in many states, and Democrats had not adopted especially effective or extreme gerrymanders a decade earlier. Overall, Democrats eliminated 12 seats that leaned Republican in the last presidential election in New York, Illinois, New Mexico, Nevada and Oregon. No state courts have acted to weaken Democratic gerrymanders in those states.

Republicans, on the other hand, have faced a string of adverse court rulings.

In Ohio, North Carolina and Pennsylvania, the courts either limited Republican gerrymanders or selected surprisingly Democratic-leaning maps. In the end, Republicans may only eliminate a handful of Democratic districts, like those in the northern suburbs of Atlanta, Nashville and, perhaps soon, eastern New Hampshire.

The relatively balanced congressional map could be short-lived, however.

While courts increasingly claim that it is too late to challenge maps before this year’s primary season, cases in Pennsylvania, Georgia, Alabama and other states could be reconsidered after the midterm elections. The court-drawn map in North Carolina will also be redrawn after this year’s election — and possibly after an additional Republican wins a seat on the state’s Supreme Court. Other states could choose to redraw the maps mid-cycle, if one party wins control in this year’s midterm elections.

For now, the redistricting cycle counts as a rare — if relative — bright spot for American democracy.

Despite gerrymandering, the House of Representatives will again be the most representative and democratic body of the federal government: The place where the party that wins the most votes is likeliest to win power. In recent decades, Democrats have experienced the exact opposite — routinely failing to win power despite winning the most votes. That history has raised fears of minority rule, where one party might win full control of government without a majority of the vote.

That is still a possibility in the Senate and in the Electoral College, where Republicans continue to possess considerable structural advantages. But now, because of the relative fairness of the map, that is a lot less likely in the House than it’s been in a long time.