Joe Biden and Donald Trump may have secured their parties’ presidential nominations, but there is plenty of intrigue further down the ballot heading into the next primary day Tuesday.

While much of the focus will be on Ohio’s Senate race, where Republicans are competing to take on Democratic Sen. Sherrod Brown, Tuesday’s results will set the stage this fall for pivotal House battleground races in the Buckeye State and in Illinois. Two incumbents in Illinois are also facing competitive primary challenges in safe seats. And in California, voters will decide whether Republicans’ narrow House majority will receive an immediate jolt.  

Here are four dynamics to watch in Tuesday’s House primaries.

A test for Trump’s endorsement

Trump’s endorsement will be put to the test in Illinois’ 12th District, where his preferred candidate, GOP Rep. Mike Bost, is facing a primary challenge from former state Sen. Darren Bailey. 

Bailey had Trump’s endorsement in the 2022 gubernatorial race, when he lost to incumbent Democrat J.B. Pritzker by 13 percentage points. But last month, Trump instead backed Bost, who leads the House Veterans’ Affairs Committee, writing on his Truth Social platform, “While I like and respect Darren Bailey, and was proud to campaign for him in 2022, Mike Bost was one of the first House Committee Chairmen to endorse my Campaign, and Mike was a stalwart supporter of our America First agenda during my record-setting Administration.”

Bost has outspent Bailey in the race as he’s touted Trump’s endorsement on the airwaves, launching one TV ad featuring footage of the former president praising the five-term congressman.  

Bailey, meanwhile, has argued that Bost is not sufficiently conservative. Bailey aired one ad focused on the congressman’s votes on immigration and support for aid to Ukraine, suggesting that Bost did not support funding for Trump’s proposed wall at the southern border. Bailey also rallied with Rep. Matt Gaetz, R-Fla., last month.

Longtime Democrat faces primary rematch

Longtime Rep. Danny Davis, D-Ill., is once again facing a primary challenge, including from one candidate who nearly defeated him in 2022. 

Community organizer Kina Collins lost to Davis by just 6 points in 2022 in the Democratic primary for Illinois’ 7th District. But this year she’s drawn the ire of United Democracy Project, a super PAC tied to the pro-Israel group AIPAC. 

United Democracy Project has spent $210,000 on ads against Collins, who supports a cease-fire in the Gaza Strip, although the group’s ads focus on crime and police funding, not the ongoing war between Israel and Hamas. 

The anti-Davis vote could be split this year between Collins and three other Democrats, including Chicago city Treasurer Melissa Conyears-Ervin. She has outraised Davis in the race, although the 14-term congressman had more money left in his campaign account as of Feb. 28. 

Battleground matchups for Democrats on defense

Any close race could determine which party controls the House after November’s elections, given the GOP’s narrow majority. And there are three races across Ohio and Illinois that are expected to be serious battlegrounds this fall. 

The most contentious has been the GOP primary for Ohio’s 9th District, a race to decide who will take on longtime Democratic Rep. Marcy Kaptur. Trump won the Toledo-area district in 2020, according to data compiled by Daily Kos, but Kaptur has long overperformed. 

Republicans had been worried that Kaptur’s last opponent, J.R. Majewski, could repeat his poor performance if he won the nomination. But Majewski dropped out of the primary this month, after publicly wavering for days, leaving two major candidates in the race: state Rep. Derek Merrin and former state Rep. Craig Riedel. 

Riedel faced criticism from key conservative lawmakers after audio surfaced of him criticizing Trump, and Merrin’s allies have linked Merrin to the former president on issues like immigration. Merrin’s tied himself to Trump, and the former president endorsed him on the eve of the primary.

But Riedel is pushing back by casting Merrin as a “never Trumper” for not answering a question about who he’d vote for in the presidential primary. Riedel is also trying to tie Merrin, a prominent state lawmaker and committee chairman, to the scandal that brought down the state’s former House speaker and other major Republicans.

Then there’s Ohio’s 13th District, where former state Sen. Kevin Coughlin and Hudson City Councilman Chris Banweg are vying for the Republican nomination to take on freshman Democratic Rep. Emilia Sykes. 

Republicans are eying this Akron-area seat, which Sykes won by 5 points in 2022 and Biden won by 3 points in 2020 (according to Daily Kos’ data). Sykes is expected to lean on her experience in the area as the former top Democrat in the state House, as well as on reproductive rights in a state where voters just codified a right to abortion into the state constitution.

Illinois’ 17th District, a sprawling district across the northwestern portion of the state along the border with Iowa, may be safer ground for Democrats. Unlike the two Ohio races, which The Cook Political Report With Amy Walter, a nonpartisan election forecaster, rates as “toss-ups,” Democratic Rep. Eric Sorensen’s seat is considered “lean Democrat.” But while Biden won the seat by 8 points in 2020, per Daily Kos, Sorensen, a longtime meteorologist in the area, won the seat by 4 points in a tough midterm, open-seat fight.

The top Republican in the primary is Joe McGraw, a former prosecutor and judge who’s heavily emphasizing criticism about Democratic border policies, following the strategy that Republicans are ready to deploy across the country this cycle. His top opponent is farmer Scott Crowl. The winner will start at a significant cash disadvantage, as Sorensen had more than $1.7 million banked away through the end of February to McGraw’s $226,000 and Crowl’s just $7,000. 

Who will join Congress this year, and when?

It seems likely that Republicans will ultimately win the special election for California’s 20th District, which was vacated following former Speaker Kevin McCarthy’s resignation. But the margin will be important, particularly for Republicans in Washington struggling with a razor-thin majority. 

The top candidates for the seat faced off in a regularly scheduled primary just weeks ago, but Tuesday’s special primary election will decide who will serve out the rest of McCarthy’s vacated term this year. Candidates from all parties compete on the same ballot, and if one of them wins a majority, they’ll win the seat outright. That means they’d be able to join Congress almost immediately, instead of having to wait until the results of a late May special election. 

Republican state Assemblyman Vince Fong, who won 41% of the vote in the normal primary, is the favorite here. If he can improve on that vote share and win a majority of an electorate that’s expected to be much smaller, he can head to Washington within days or weeks. But if not, he’d head to that May 21 runoff, likely against either Republican Sheriff Mike Boudreaux or Democratic teacher Marisa Wood, the only other candidates to finish with double digits in this month’s primary.

In Ohio’s 6th District, there’s also a special primary election Tuesday, after former Rep. Bill Johnson left Congress at the beginning of the year to run Youngstown State University.

State Sen. Michael Rulli, backed by U.S. Rep. David Joyce of Ohio and former state GOP Chair Jane Timken, and state Rep. Reggie Stoltzfus, who has been backed by former House Freedom Caucus Chairman Scott Perry of Pennsylvania as well as far-right former Trump national security adviser Michael Flynn, are running. So is Rick Tsai, a chiropractor from East Palestine who says he’s running because of his frustration with the handling of the train derailment.