Democrats hope to ride a wave of liberal enthusiasm and anti-Trump sentiment to a House majority in the midterm elections in November. All 435 House seats are up for grabs, but only about 48 seats are considered competitive (rated tossup or leaning toward one party), based on an average of estimates from three organizations.

Democrats, who have been sidelined as the House minority party since 2010, need to flip 24 Republican seats while keeping the 194 seats they currently hold.

Two indicators that are considered good predictors of the midterm vote have given Democrats hope. First, the party is performing better than Republicans in generic ballot polls — surveys in which people are asked which party they would support in a congressional election. Second, President Trump’s approval rating is relatively low. But congressional races are inherently local elections, and the individual candidates on the ballot will matter in November.

The math favors the Democrats: 41 of these competitive seats are held by Republicans, while only 7 are held by Democrats. Most of these races are in districts where Hillary Clinton and Mr. Trump barely outperformed the other in the 2016 presidential race, and many of the incumbents won with smaller margins as well.

Even some Republicans seats in Trump country are in jeopardy. On March 13, Conor Lamb, a Democrat and former Marine, won a razor-thin upset in a special House election in southwestern Pennsylvania, in a district that Mr. Trump won by nearly 20 percentage points in 2016.

Democrats will surely look first to flip the 25 districts where Mrs. Clinton outperformed Mr. Trump. Six are leaning toward Democrats, including Martha McSally’s open seat in Arizona’s Second District. Two, however, are likely to remain Republican, including New York’s 24th District, where John Katko won 60 percent of the vote in 2016 and is unlikely to face a strong Democratic challenger.

Fund-raising plays a major part in elections, and several Republican incumbents are being outraised by Democratic challengers. Several of these districts are still considered safely Republican, but some, like New York’s 19th — where Antonio Delgado had raised $287,000 more in 2017 than the incumbent, John Faso — are viewed as competitive because of the Democrat’s fund-raising advantage.

Republicans can still go on the offensive and will look first to flip seats in districts that voted most strongly for Mr. Trump. Two of the 12 Democratic districts, both of which Mr. Trump won easily, are rated as tossups.

But half of the seats in these districts are solidly or likely Democratic, and Trump voters might not turn out for Republicans without the president on the ballot.

More than 30 Republicans have announced that they will not be running for re-election in their district, a far greater number than in previous years. But most of those races are still rated solidly or likely Republican, so they are unlikely to flip to a Democrat.

However, six of the Republican seats are currently leaning toward the Democrats, including Darrell Issa’s southern California seat, which he narrowly won in 2016, and Ileana Ros-Lehtinen’s Miami-area seat where Mrs. Clinton carried the vote by more than 19 percentage points.

Last month, the Pennsylvania Supreme Court released new district boundaries after ruling the state’s gerrymandered map unconstitutional. Over all, the new map is great news for Democrats because it pushed multiple districts to the left, inching the party closer to the 24 seats needed to retake the House.