WASHINGTON — The 2024 battle for control of Congress centers on just a handful of Senate races and about two dozen House seats, putting considerable pressure on those candidates to win over voters as party leaders and super PACs funnel millions of dollars into their campaigns.

The incumbents representing those states and congressional districts will spend nearly all of their time campaigning between now and Election Day, with Congress in session just three weeks ahead of Nov. 5. They’ll be fighting off challengers who will be on the home front the entire time.

Republicans aim to increase their slim majority in the House of Representatives and flip the Senate, while Democrats are hoping to hang onto their majority in the upper chamber and regain control of the House.

Experts interviewed by States Newsroom said the outcome will be determined by multiple factors, including turnout, ticket splitting and the trajectory of the presidential campaign, which underwent an abrupt change with the exit of President Joe Biden and the nomination of Vice President Kamala Harris as the Democratic candidate.

At stake is whether Harris or Donald Trump, the Republican presidential nominee, faces a Congress friendly to their ambitions or another two years of a deeply divided government in the nation’s capital. Biden has struggled with a GOP House and a Senate narrowly controlled by Democrats during the past two years.

“There’s a lot of energy on both sides for these congressional races, because of just how close the margins are going to be in the House and Senate,” said Casey Burgat, assistant professor and legislative affairs program director at George Washington University.

Senate GOP control?

The Senate is trending toward a Republican majority, though that will be determined by voters in Michigan, Montana, Nevada and Ohio, all of which are considered toss-ups by the Cook Political Report with Amy Walter, a respected non-partisan publication.

If the GOP picks up any one of those four seats, that would give Republicans at least a 51-seat majority in the chamber after winning the West Virginia seat that’s currently held by Joe Manchin III, thought to be nearly certain in the GOP-dominated state.

Democrats maintaining those four seats as well as three others classified as “lean Democrat” by the Cook Political Report would leave control of a 50-50 Senate to the results of the presidential election, since the vice president casts tie-breaking votes in that chamber.

The Senate map is highly favorable to Republicans, who are defending 11 seats in safely red states, while Democrats are trying to hold on to 23 seats, with seven of those in purple states.

While the entire 435-member House of Representatives is up for reelection every two years, senators are elected to six-year terms, leaving about one-third of the chamber up for reelection in evenly numbered years.

Eyes on Montana

Robert Saldin, professor of political science at the University of Montana, said during an interview that Democratic Sen. Jon Tester will need to get voters in the state who support Trump to split their tickets if Tester is going to secure reelection.

“One of the secrets to Tester’s success over the years is that he has been able to distinguish himself from stereotypes of the national Democratic Party,” Saldin said. “And that’s going to be really important again, obviously, because Trump is on the ticket, and is certainly going to carry the state by a very wide margin.”

Republican challenger Tim Sheehy could potentially have a bit of an easier time getting elected this November in the deeply Republican state, he said.

“All he has to do is get people who are voting for Trump to also vote for him. And in fact, he can probably lose some tens of thousands of voters and still be okay,” Saldin said.

Sheehy, however, is somewhat disadvantaged by not having run in a competitive GOP primary, leaving him to move directly into a high-stakes general election.

“He is a political novice,” Saldin said. “He didn’t have a practice run in the primary. And here he is fresh out of the gate in one of the most expensive, most watched, most hyped Senate elections in the country. And so he’s having to learn as he goes.”

One factor that could benefit Tester over Sheehy is a ballot question addressing abortion access in the state that is likely to go before voters in November, Saldin said.

“That should give at least a little nudge in the direction of the Democrats,” Saldin said.

A dozen other states have approved or could approve abortion ballot questions, including Arizona, Florida, Maryland and Nevada.

In Ohio, edging away from national politics

Paul A. Beck, academy professor of political science at The Ohio State University, said Democratic Sen. Sherrod Brown has sought to separate himself from national political figures throughout this reelection bid.

“I think he really wants to make this a local contest, a statewide contest, not a national contest,” Beck said. “And so he’s going to do everything he can to not appear on the stage with the Democratic nominees for president, and everything he can to try to define himself as somebody who is above partisan politics.”

Brown was elected to the House in 1992 before winning election to the Senate in 2006. He secured reelection in 2012 and 2018, though Republican candidate Bernie Moreno is looking to end that streak this year.

Beck said a ballot question about redistricting could help boost turnout, potentially increasing Brown’s chances.

“It’s going to energize voters and is going to produce higher turnout on the left than it will on the right, and that could be a factor in 2024,” Beck said.

Incumbent clout

Sitting members of Congress have historically held an advantage that may help the party that holds more members seeking to return to Capitol Hill for the 119th Congress.

“Reelection rates in the House have never dropped below 85%, and have recently stretched to highs of 98% in 2004 and 97% in 2016,” according to analysis from Miro Hall-Jones at OpenSecrets, a nonpartisan organization that reports on the role of money in U.S. politics. “While the Senate is more susceptible to shifts in public opinion — reelection rates dropped as low as 50% in 1980 at the dawn of the Reagan Revolution — incumbent senators have retained their seats in 88% of races since 1990.”

Every single one of the 28 senators seeking reelection two years ago was able to convince voters in their home states to give them another term, marking the first time that happened in American history, according to the analysis.

Michigan and Arizona Senate races could then present a bigger challenge for Democrats, since both those seats are open due to the upcoming retirements of Michigan Sen. Debbie Stabenow and Arizona independent Sen. Kyrsten Sinema.

Michigan is currently rated as a toss-up by the Cook Political Report while Arizona is rated as “lean Democrat.”

In the House, the 11 seats held by Republicans and considered by the Cook Political Report to be toss-ups all are held by an incumbent seeking reelection, while two of the Democrats’ 11 toss-up seats are open.

House majority

The contest for who controls the House after November is “razor thin,” said Burgat from George Washington University. With Republicans holding on by a slim margin, Democrats only need a net gain of four seats to capture the majority.

“Whether it goes Democratic or Republican, it’s not going to be by much,” Burgat said.

While Democrats and Republicans will focus much of their attention on the 22 toss-up races, they’ll also be funneling resources toward the campaigns that are rated as only leaning in their direction, as opposed to being likely or solid seats.

The Cook Political Report has eight seats held by GOP lawmakers as leaning in Republicans’ favor while 14 races, all Democratic-held seats except one, are in districts that “lean Democrat.”

That makes a total of about 44 competitive House races, according to the Cook Political Report’s analysis.

Among the closely watched congressional districts:

• Both parties are eyeing open seats in California, Colorado, Michigan and Virginia.

• Michigan’s 7th and 8th districts, respectively opened by Democrats Elissa Slotkin, who is seeking the state’s open Senate seat, and the retiring Dan Kildee, are rated by the Cook Political Report as toss-ups.

• Virginia’s 7th Congressional District, being vacated by Democrat Abigail Spanberger, who will focus on a run for governor, is expected to lean blue.

• California Democratic Rep. Katie Porter’s district also leans towards Democrats. Porter decided to leave the seat for an unsuccessful Senate run.

• Colorado’s 3rd, abandoned by Republican Lauren Boebert, who shifted to a friendlier district, leans Republican, according to the Cook Political Report.

• One open seat each in Maryland and New Hampshire is rated as likely Democrat by the prognosticator.

DCCC ads

The Democratic Congressional Campaign Committee is throwing big money where races are competitive, zeroing in on 15 media markets including those in Arizona, Colorado, Michigan, North Carolina and Pennsylvania. The official campaign arm of House Democrats announced a $28 million initial ad buy in June, spending double on digital ads compared to 2022, according to the organization.

The DCCC maintains that House Democrats “have always had multiple paths to reclaim the majority in November — including our 27 Red to Blue candidates in districts across the country working to defeat extreme Republicans who are out-of-touch with their communities,” spokesperson Viet Shelton told States Newsroom in a written statement.

Shelton said voters are “fed up with these politicians who are more interested in obeying Trump, voting for abortion bans, and giving tax breaks to billionaires and big corporations, while ignoring the needs of the middle class.”

The DCCC sees pickup opportunities in the 16 districts Biden won in 2020 that are currently held by Republicans.

Among them are five seats in California, four in New York, two in Arizona, and one respectively in New Jersey, Oregon, Pennsylvania and Virginia. Nebraska also saw incumbent GOP Rep. Don Bacon’s district go to Biden.

The switch to Harris at the top of the Democratic ticket could boost challengers in heavily college-educated districts, according to the Cook Political Report’s latest analysis of the field.

Those include Arizona’s 1st Congressional District held by GOP Rep. David Schweikert, New Jersey’s 7th occupied by freshman Thomas Kean Jr., and New York’s 17th held by Mike Lawler, also in his first term. All are rated as Republican toss-ups by the Cook Political Report.

Frontline candidates

But the DCCC campaign has vulnerable Democratic incumbents to worry about as well. The organization has identified 31 as “frontline” members, meaning their purple districts are what campaigners describe as “in play.”

Among them is 40-year Democrat Marcy Kaptur, who has held Ohio’s 9th Congressional District, in the state’s northwest region, since 1983.

Other vulnerable Democrats include Reps. Yadira Caraveo in Colorado’s 8th, Jared Golden in Maine’s 2nd, Don Davis in North Carolina’s 1st, Gabe Vasquez in New Mexico’s 2nd, Emilia Sykes in Ohio’s 13th, Susan Wild and Matt Cartwright in Pennsylvania’s 7th and 8th districts, and Marie Gluesenkamp Perez in Washington’s 3rd.

The National Republican Congressional Committee did not respond to requests for comment, but the House GOP campaign arm announced in late June a $45.7 million initial ad buy across 29 media markets, with large chunks going to metro areas in Los Angeles; New York City; Albuquerque, New Mexico; Anchorage, Alaska; Denver, Colorado; Detroit, Michigan; Portland, Oregon; and Omaha, Nebraska.

The ad buy — an offensive to “to grow our majority,” NRCC Chair Richard Hudson said in a press release — will specifically target 13 districts currently held by Democrats.