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.

Ratings for Each House Race Democratic Republican Solid Likely Lean Tossup Lean Likely Solid 182 7 10 22 16 26 172

Note: There are currently five vacancies in the House whose seats are placed based on their past occupants: Arizona’s Eighth, Ohio’s 12th and Pennsylvania’s 14th (the old 18th district) are shown as Republican seats, while Michigan’s 13th and New York’s 25th are shown as Democratic seats. *Each of Pennsylvania’s districts, which were redrawn last month , is represented by the new district for that area and the new district’s race rating.Note: There are currently five vacancies in the House whose seats are placed based on their past occupants: Arizona’s Eighth, Ohio’s 12th and Pennsylvania’s 14th (the old 18th district) are shown as Republican seats, while Michigan’s 13th and New York’s 25th are shown as Democratic seats.

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.

Most competitive races are in Republican districts.

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 Republican 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.

Competitive districts Democrat-held Republican-held

These competitive districts are found from Maine to California, in red and blue states, in cities and in rural areas.

Democrats are likely to target the 25 G.O.P. districts that voted for Clinton, plus the 12 red districts where they’ve raised the most money.

There are 25 Republican districts where Hillary Clinton outperformed Donald J. Trump in the 2016 presidential election and 12 districts where a Democratic candidate had raised the most money in 2017 in an upcoming House race, excluding districts with special elections.

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 are likely to target Democrats in Trump country.

There are 12 Democratic districts where Mr. Trump outperformed Mrs. Clinton in the 2016 presidential election.

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.

Many Republican representatives have chosen not to seek re-election, but most of the open seats are tough to flip.

All Open House Seats Vacant seats, which will be filled by special elections, are not included in the list of open seats.

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.

Competitive Open Seats District Retiring Rep. 2016 pres. margin Kan. 2 L. Jenkins (R) R+18 Lean Rep. Minn. 8 R. Nolan (D) R+16 Tossup Minn. 1 T. Walz (D) R+15 Tossup Mich. 11 D. Trott (R) R+ 0 4 Tossup N.J. 11 R. Frelinghuysen (R) R+ 0 1 Tossup Wash. 8 D. Reichert (R) D+ 0 3 Tossup Calif. 39 E. Royce (R) D+ 0 9 Tossup N.J. 2 F. LoBiondo (R) R+ 0 5 Lean Dem. N.H. 1 C. Shea-Porter (D) R+ 0 2 Lean Dem. Nev. 3 J. Rosen (D) R+ 0 1 Lean Dem. Pa. 7* C. Dent (R) D+ 0 1 Lean Dem. Ariz. 2 M. McSally (R) D+ 0 5 Lean Dem. Nev. 4 R. Kihuen (D) D+ 0 5 Lean Dem. Calif. 49 D. Issa (R) D+ 0 8 Lean Dem. Fla. 27 I. Ros-Lehtinen (R) D+20 Lean Dem. *Pennsylvania districts are from the state’s new map . Representative Charlie Dent currently represents the old 15th District, which most closely resembles the new Seventh District.

Democrats will also have to defend 16 open seats. Representatives in the five most competitive races won by fewer than five percentage points in 2016; three of them earned less than 50 percent of the total vote.

Democrats got some unexpected help from a new congressional map in Pennsylvania.

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.

District Rating Old New Old New 0 3 16 Solid Rep. Solid Rep. 0 5 15 Solid Rep. Solid Rep. 0 9 13 Solid Rep. Solid Rep. 10 12 Solid Rep. Solid Rep. 11 0 9 Solid Rep. Solid Rep. 0 4 10 Solid Rep. Likely Rep. 12 17 Solid Rep. Tossup 16 11 Likely Rep. Solid Rep. 18 14 Lean Rep. Solid Rep. 0 8 0 1 Lean Rep. Tossup 15 0 7 Lean Rep. Lean Dem. 0 6 0 6 Lean Rep. Likely Dem. 0 7 0 5 Lean Dem. Solid Dem. 17 0 8 Likely Dem. Likely Dem. 0 1 0 2 Solid Dem. Solid Dem. 0 2 0 3 Solid Dem. Solid Dem. 13 0 4 Solid Dem. Solid Dem. 14 18 Solid Dem. Solid Dem.

Flipping 24 seats is not unprecedented, or even uncommon.

There have been swings of 24 seats or more in half of the midterm elections since 1994. The last time Democrats took the House with that kind of swing was the 2006 midterm election, when they picked up 32 seats. In 2010, as the Tea Party movement against President Obama’s policies shifted the political landscape, Republicans took control of the House by flipping 64 seats. They have held the chamber ever since.