The most important Senate primary of the year is just one of today’s elections

Donald Trump is trying to drag a few allies across the finish line on Tuesday.

The former president is himself certain to cruise to easy victories in the five states holding presidential primaries, but his power will be tested in a handful of down-ballot races.

That includes Ohio Senate candidate Bernie Moreno, who has spent the final days of the closely contested GOP primary trying to move quickly past an Associated Press story about an account on an adult website that he says was created by an intern in 2008. Or five-term House incumbent Mike Bost, who’s brandishing Trump’s endorsement as a shield against a MAGA-aligned challenger in Southern Illinois.

The presidential primaries will also provide a key signal about the November rematch between Trump and President Joe Biden. Though both men have already mathematically clinched their respective parties’ nominations, the campaigns will be monitoring Tuesday’s results for the latest check-in on the holdouts: voters who still aren’t on board with their party’s nominee and in November could cross over, vote third-party or stay home.

There’s even more to watch down-ballot: Tuesday’s primaries also represent the latest House GOP tug-of-war between institutionalists and insurgents, with two safely GOP open seats in Ohio and the special election in California to replace former House Speaker Kevin McCarthy. And it's the latest Democratic skirmish over the Israel-Hamas war, with the most prominent pro-Israel group seeking to exert its influence on a Democratic primary in Chicago after floundering in a similar race in Southern California earlier this month.

Here are five things to watch in Tuesday’s primaries:

The power of Trump’s endorsement

The former president is a prolific endorser in Republican primaries, but four races on Tuesday offer unique tests of his hold on the party.

Moreno is locked in a competitive, three-way race for the nomination to face Democratic Sen. Sherrod Brown in what could be the decisive contest for control of the Senate. There’s little in the way of reliable, independent polling, though most observers see Moreno and self-funding state Sen. Matt Dolan — who is running in a more moderate lane — as the top contenders, with Secretary of State Frank LaRose a tier below.

Trump is putting his political capital behind Moreno, trekking to Ohio for a weekend rally just 48 hours after the AP story was published. He reportedly scheduled the trip the week before the primary after polling showed Moreno had not locked down the nomination despite Trump’s long-standing endorsement.

In addition to the Senate race, the presumptive GOP presidential nominee is also endorsing three House candidates in competitive primaries.

His backing of Bost over 2022 Illinois gubernatorial nominee Darren Bailey was reportedly at the behest of House Speaker Mike Johnson and Rep. Richard Hudson (R-N.C.), the chair of House Republicans’ campaign arm. The endorsement put Trump crosswise with some of his allies, like Rep. Matt Gaetz (R-Fla.), who are supporting Bailey. Bailey is running to Bost’s right, so the vote will test whether the most conservative voters support the more conservative candidate — or if Trump’s imprimatur is enough to keep them in the fold.

Trump also made a last-minute endorsement on Monday in one of House Republicans’ most important primaries: the race to face vulnerable Democratic Rep. Marcy Kaptur in Northwest Ohio. The former president is backing state Rep. Derek Merrin, who has also been the beneficiary of the Johnson-linked Congressional Leadership Fund super PAC.

But whether Merrin beats former state Rep. Craig Riedel on Tuesday, House Republicans have already avoided their nightmare scenario. With help from Trump, 2022 nominee J.R. Majewski, who was defeated soundly by Kaptur, dropped out of his second shot at the seat earlier this month and endorsed Merrin. The seat will be one of the most pivotal to control of the closely divided House.

And out in California, Trump is supporting state Rep. Vince Fong in the special election to replace former House Speaker Kevin McCarthy. Fong, who also has McCarthy’s endorsement, was the first-place finisher in the regular primary with 42 percent of the vote.

There are fewer candidates on the ballot in the special election, and Fong has a real chance to win a majority of the vote. Otherwise, he’s likely to face a fellow Republican, Tulare County Sheriff Mike Boudreaux, in May — in addition to their already-scheduled November showdown.

The battle for the Senate

It’s only mid-March, but Tuesday’s Ohio Senate primary will resolve the biggest question mark on the entire Senate map.

Ohio is a pivotal race: It’s one of three Trump-won states Democrats are defending in November, and because the seat in neighboring West Virginia has all but slipped away with Democratic Sen. Joe Manchin’s retirement, the party has no margin of error.

Republicans can clinch control of the chamber with just one more seat, whether it’s Brown’s in Ohio’s, Democratic Sen. Jon Tester’s in Montana — or by winning any number of battleground-state races, like an open seat in Michigan, Democratic Sen. Jacky Rosen’s seat in Nevada, or by toppling Democratic Sens. Bob Casey in Pennsylvania or Tammy Baldwin in Wisconsin.

And it's worth remembering that Republicans are operating at a big advantage. A victory in the presidential race would also give Republicans control of the Senate after Inauguration Day, even if West Virginia is the only seat that changes hands in November.

That's perhaps why Democrats meddled in the race in the closing week of the campaign. They're boosting Moreno, apparently seeing him as an easier candidate to beat in November.

Either way, though, Brown has a tough race ahead of him in a state Trump won twice by 8 percentage points.

The direction of the House Republican conference

The volatile House Republican conference will be getting at least two new members picked on Tuesday, thanks to open, solidly red seats in Ohio. Those races, to replace retiring Rep. Brad Wenstrup and former Rep. Bill Johnson, are the next skirmishes between the party’s establishment and disruptive blocs.

There are 11 candidates running to replace Wenstrup in Southern Ohio, and the winner could prevail with just a plurality of the vote. Only four of the candidates have run TV ads, but candidate fields that large can be volatile.

House GOP leaders are keenly aware of these primaries and their potential impact on how manageable or unruly the conference will be — it’s part of why Johnson and Hudson pushed Trump to endorse Bost in Illinois, or Fong in California.

The first congressional primaries on Super Tuesday already resulted in a few nominees who seem poised to be thorns in the side of leadership, like Brandon Gill, the 30-year-old son-in-law of conservative commentator Dinesh D’Souza. Gill won a primary in North Texas despite more than $2 million in outside spending from establishment-aligned groups.

Each party’s presidential holdouts

Functionally, the presidential primaries are over: Both Trump and Biden have secured enough delegates to be their parties’ respective nominees.

But below the surface, there will still be clues for November. Take Arizona, which is holding its presidential primaries on Tuesday. The state has a slice of moderate Republicans who have been decisive in the past — voters who carried candidates like former Sen. John McCain and former Gov. Doug Ducey to victory, while rejecting Trump in 2020 and Kari Lake in 2022.

Do these voters — many of whom are still registered Republicans — register a protest vote against Trump by supporting one of his former rivals whose names are still on the ballot?

On the Democratic side, activists protesting Biden’s handling of the Israel-Hamas war are encouraging their supporters to vote for Marianne Williamson, as the state does not have an “uncommitted” option.

And this is mostly a curiosity: Florida Gov. Ron DeSantis’ name is on the ballot in his home state’s primary, since he was still an active candidate last December when the deadline to withdraw from the primary passed. How many votes will he draw?

The Dem divide over Israel

The Israel-Hamas war has become the most significant divide in the Democratic Party, including in a handful of primaries in Illinois.

In Chicago, 82-year-old Democratic Rep. Danny Davis faces two prominent challengers — one of whom, activist Kina Collins, has drawn negative ads from United Democracy Project, the super PAC funded by the American Israel Public Affairs Committee.

It’s the group’s second significant effort in Democratic primaries this year, coming two weeks after a more expensive campaign for the open congressional seat currently held by Democratic Rep. Katie Porter in Orange County, California, flopped — state Sen. Dave Min easily secured a spot in the general election despite United Democracy Project’s $4.7 million in spending against him.

The super PAC has spent just under a half a million dollars against Collins, including a TV ad that hits her for supporting the “defund the police” movement in 2020.

Two other Democratic incumbents in suburban Chicagoland districts are facing opponents who have made the war a central part of the campaign: Reps. Sean Casten and Bill Foster. But both are considered to be on safer ground than Davis.