HARRISBURG, Pa. — Since he became the single biggest force in Republican politics nearly a decade ago, Donald Trump has figured prominently in U.S. Senate races, using his massive public platform and loyal base of support to shape which GOP incumbents run for reelection and which contenders get nominated.

This year, with control of the U.S. Senate on the line, would-be nominees in the Senate battlegrounds of Arizona, Montana, Michigan, Nevada and Ohio are endorsing Trump in the GOP presidential primary, campaigning for him or otherwise seeking his approval.

But that's not happening in Pennsylvania, where an awkward dance between likely partners at the top of the ticket seems all but certain to continue until the music stops in November.

There, the likely Republican nominee, David McCormick , and Trump seem to be ignoring each other. Theirs is a complicated relationship that Democrats say is fraught with danger for McCormick and pollsters say makes his uphill climb against an entrenched incumbent much steeper.

“It’s a fascinating relationship to observe,” said Christopher Borick, director of the Muhlenberg College Institute of Public Opinion in Allentown. “They’re not a natural fit in terms of their policy positions, their rhetorical styles and, of course, given Trump’s support of one of McCormick’s opponents in 2022, there is a history. And how do you navigate that becomes the question.”

McCormick — a wealthy ex-hedge fund CEO and onetime senior official in President George W. Bush’s administration — has no major primary opposition, is popular with party brass and doesn't need Trump's fundraising help.

But he is trying to knock off Democratic U.S. Sen. Bob Casey , the son of a former governor and the best-known political name in Pennsylvania.

For Democrats, Pennsylvania is a must-win if they are to somehow hang on to Senate control and help President Joe Biden remain in the White House. Biden's unpopularity — even in Pennsylvania, where Biden was born, roots for Philadelphia sports teams and campaigns often — rivals Trump's, and the presidential race will weigh heavily on both McCormick and Casey, one of Biden's strongest allies in Congress.

For McCormick, there’s the additional baggage of Trump's unpopularity with the moderate voters McCormick wants to win over — not to mention the tongue-lashings Trump administered as he worked to defeat McCormick in Pennsylvania’s hotly contested 2022 primary for U.S. Senate.

McCormick — like others in the seven-way GOP primary to succeed retiring Republican Sen. Pat Toomey — had sought Trump's endorsement. According to McCormick’s telling of it, Trump told McCormick during their meeting at Trump's Mar-a-Lago residence in Florida that to win the primary McCormick would need to say the 2020 election was stolen.

McCormick said he refused. Three days later, Trump endorsed Dr. Mehmet Oz and then savaged McCormick repeatedly.

In one setting, a rally in western Pennsylvania days before the 2022 primary, Trump told the crowd that McCormick is “not MAGA,” using the acronym for his Make America Great Again campaign slogan.

Then he derided McCormick as having been with a company — the hedge fund — that “managed money for communist China," describing him in the next breath as "the candidate of special interests and globalists and the Washington establishment.”

McCormick lost to Oz by just 950 votes — a loss he has acknowledged that Trump probably contributed to — before Oz went on to lose to Democrat John Fetterman by 5 points.

Still, McCormick's allies say he hasn’t harbored hard feelings, and he hasn’t since criticized Trump for that episode — or anything else, really.

But Trump has not walked back his previous comments — and that could hang over McCormick.

“It’s definitely a problem for McCormick,” said Pittsburgh-based Democratic campaign strategist Mike Mikus. “The question is, how big? Donald Trump is always the wild card in these types of relationships. Donald Trump cares about one person and that’s Donald Trump, and if he’s not feeling it one day when he's in Pennsylvania, he may start taking shots at McCormick.”

McCormick declined an interview request, and Trump’s campaign aides didn't respond to messages. But the pall between them is hard to miss.

The men haven’t spoken to each other since 2022, according to McCormick’s campaign. They didn't meet when Trump was in the state recently to speak to National Rifle Association members at the NRA’s Great American Outdoor Show. McCormick didn't attend and Trump never mentioned McCormick during the 82-minute speech.

In a recent interview with conservative broadcasters, McCormick acknowledged the likelihood that the men will lead the GOP ticket in Pennsylvania and described the relationship in transactional terms.

“My guess is that President Trump at the top of the ticket will help me, and I’d be hopeful that my candidacy and the strength I would bring will help him,” McCormick said on “The Clay Travis & Buck Sexton Show.”

McCormick tiptoed around Trump’s liabilities with moderate voters in a state Trump lost by 1 point in 2020. Instead, McCormick suggested Trump can help him because he mobilizes a huge part of the Republican Party to vote — and that McCormick can help Trump with more moderate voters.

McCormick has said he hadn't believed he needed Trump’s endorsement to win in 2022, as long as Trump wasn't attacking him, and acknowledged the necessity of convincing moderate voters to back him. On Thursday, he name-dropped Trump when he told the crowd at a family farm in northern Pennsylvania that he is wealthy and, thus, can be an independent politician without fear of losing the job.

“I don’t need to make a living after this,” McCormick said. “I don’t owe anybody anything. I don’t owe President Trump anything. I don’t owe (Senate GOP Leader) Mitch McConnell anything. The only people I would owe anything to are the people of Pennsylvania who put me in office.”

Still, Borick and other pollsters aren’t sure if there’s a needle left for McCormick to thread, given Trump’s unpopularity with moderates and the Supreme Court’s decision to overturn Roe v. Wade, ending a half-century of federal protection of abortion rights.

McCormick is an abortion opponent — he has said most recently that he supports letting states ban abortion, with exceptions for rape, incest and to save the life of the mother — a position that could limit his appeal to otherwise persuadable moderates.

On top of that, McCormick is virtually unknown compared to Casey, pollsters say.

“Right now, the biggest vulnerability for McCormick is nobody really knows him in this state, and he’s got to make sure he defines himself before Casey does,” said Berwood Yost, a pollster and director of the Center for Opinion Research at Franklin and Marshall College.

Yost and other analysts say it will be difficult for McCormick to win without the enthusiastic support of Trump's base. But, Yost said, McCormick may have to figure out how to do that in a challenging political environment if he is "someone who Trump doesn’t favor.”

For his part, McCormick has promised to back the GOP's presidential nominee — likely to be Trump. And McCormick has largely stuck with Trump on policy, including siding against Democrats and McConnell in the divisive fight in Congress over bipartisan legislation to tighten border security and send more aid to Ukraine to help it fight Russia's invasion.

This time around, McCormick didn't need Trump’s help to get the party's endorsement or effectively clear the primary field. And McCormick — who has deep pockets and high-level connections in business and politics — has wealthy backers in what is expected to be one of the nation’s most expensive Senate races.

A supportive super PAC reported nearly $18 million in contributions, largely from big Republican donors across the world of high finance and securities trading, and McCormick has promises of support from party brass, including a super PAC linked to McConnell.

Still, there will come a general election cycle when Trump will visit Pennsylvania again. When that happens, both men will have a decision to make about whether to appear together. That could be especially uncomfortable for McCormick, Democrats say.

“McCormick will be given a pretty bad choice: to skip it and risk potentially being a target of Trump's," Mikus said, “or going and paying the price politically for cozying up to Trump.”