Casey, a three-term senator who has built such a moderate, mild-mannered reputation he has been compared to oatmeal, is in the crosshairs of national Republican leaders. Senate Minority Leader Mitch McConnell has identified Casey as one of his top targets in 2024, going so far as to rank his seat alongside those held by Senate Democrats in solidly red states such as West Virginia and Montana.

Republicans are desperately trying to lure former business executive David McCormick, who they see as the best possible challenger, into the race. The party establishment caught a break this week when MAGA firebrand state Sen. Doug Mastriano announced that he wouldn’t run for the Senate “at this time,” potentially clearing the path to the nomination for McCormick.

But behind the scenes, GOP elected officials, strategists and donors are still not bullish about their chances against Casey. They are well aware that he has won all of his Senate elections by double digits or close to it. They know he’s an institution in the state and the son of the late anti-abortion Gov. Robert Casey, Sr. And as working-class white voters have bolted from most Democrats, GOP insiders fear Casey still has a hold on many of them, partly due to his family name.

“Bob Casey is political royalty in Pennsylvania. He’s an incumbent senator who is beloved,” said Republican Carla Sands, an ambassador to Denmark under former President Donald Trump who ran unsuccessfully for the Senate in the state last year. “I think he’s tough to beat.”

If Trump wins the presidential nomination, some Republicans worry it would make an already difficult situation all but impossible. They are concerned that Trump, who lost Pennsylvania to President Joe Biden in 2020, could be a drag on down-ballot candidates and force them to answer for his unpredictable comments. As he decides whether to run, McCormick himself is weighing the prospect of running alongside Trump, according to two Republicans close to him.

“It definitely makes it tougher for a Republican to win the Senate race if Trump is on the ticket for sure. For a Republican to beat Casey, I think the GOP presidential nominee has to win Pennsylvania,” said Josh Novotney, a Pennsylvania-based Republican strategist. “Casey is more popular than Biden in Pennsylvania and will likely outperform him.”

Though Republicans see Casey as a strong contender, the Senate map overall in 2024 heavily favors their party. Democrats are on defense in 23 states, several of them in battlegrounds and full-blown MAGA country. Republicans are facing reelection in just 11 states, with the best opportunities for Democrats lying in hostile territory like Florida and Texas. Republicans only need to flip two Senate seats — or one if they win the White House — to take back control of the upper chamber.

But the GOP’s assessment of Casey underscores another dynamic in Senate races that complicates the picture next year. Democrats have several battle-tested senators on their roster in 2024 — like Sherrod Brown, Jon Tester and potentially Joe Manchin — who are widely seen as skilled opponents and have outperformed other candidates in their party.

During the blue wave of 2018, Casey carried the state by 13 percentage points, two years after Hillary Clinton lost Pennsylvania to Trump. In 2012, Casey won by 9 percentage points, compared to then-President Barack Obama’s 5-point margin of victory.

Republicans said one challenge confronting them is that Casey is so bland that he’s not hated by anyone, a similar dilemma they faced in 2020 with Biden. Another is Casey’s constituent services.

“His staffers who represent him are very effective,” said Jackie Kulback, chair of the Cambria County Republican Party. “They’re helping with constituent issues, things like that. That helps you win elections.”

Plus, she said, “Honestly, I think there’s a lot of people who still think that they’re voting for his dad.”

The view of Casey within the GOP underscores why Republicans in both Pennsylvania and Washington, D.C. have aggressively wooed McCormick — and dissuaded Mastriano from jumping into the race. Public and private polls have shown Casey leading McCormick, but that the race would be closer if McCormick is on the ticket.

After the Senate GOP’s campaign arm took a hands-off approach to primaries last year to disappointing results, the National Republican Senatorial Committee is getting involved this time around in several states in hopes of nominating high-quality candidates. Republican officials believe McCormick, a combat veteran and former hedge fund CEO, can appeal to suburban voters and help finance what is expected to be a highly expensive campaign against Casey.

“A tough GOP opponent who is well funded could give him a race,” said Novotney.

Keith Rothfus, a former GOP congressman, is also eyeing the Senate contest. Sands, the former ambassador, recently declined to rule out a 2024 run.

Despite many Republicans’ less-than-optimistic attitude toward Casey, the Democrat is not without vulnerabilities. He has moved leftward in recent years — one liberal Philadelphia columnist dubbed him “Woke Bob Casey” — which could give an opening to Republicans to challenge his moderate image. Biden’s approval ratings are dismal and could pull down Casey and other Democrats on the ballot. Casey has voted with Biden almost 99 percent of the time, according to politics data site FiveThirtyEight, a fact that the GOP is eager to highlight. And Casey has gotten lucky, running in good years for Democrats.

In a statement Thursday after Mastriano’s announcement, McCormick signaled the lines of attack he would use on Casey were he to run.

“I am seriously considering a run for the U.S. Senate because Bob Casey has consistently made life worse for Pennsylvania families over the past 18 years, and our state deserves better,” he said. “He is openly hostile to our state’s energy industry, endorses dangerous criminals walking freely on our streets, and is enabling open borders, leading to a terrifying rise in fentanyl deaths in Pennsylvania.”

Casey’s campaign manager, Tiernan Donohue, shot back: “Bob Casey is a champion for Pennsylvania who has spent his life fighting against the ultra-wealthy and special interests that threaten Pennsylvania’s hardworking people — from countries like China that threaten our jobs, to big insurance companies that are raising our health care costs. Meanwhile, David McCormick is a Connecticut hedge fund executive who got rich outsourcing jobs, laying off workers, and cozying up to China.”

Republicans also believe that recent headlines about Casey are another liability, and that they haven’t been litigated in TV ads before. Casey’s brother registered to lobby last year and a March article by the New York Post found that Casey’s campaign paid his sister’s company for printing services. Casey’s office has said his brother will not lobby him, in accordance with Senate ethics rules.

Some in the GOP even think that Trump sharing a ticket with McCormick would be a positive for the two men. They envision a repeat of 2016, when Trump and then-Sen. Pat Toomey both won Pennsylvania, but with different paths: Toomey performed better in suburban areas, while Trump was stronger in rural parts of the state.

“To beat Bob Casey, you need to recreate the 2016 Toomey-Trump coalition,” said a national GOP strategist granted anonymity to discuss party strategy. “You blow it out in the rural areas and hold down Democrat numbers in the suburbs.”

And, some Republicans said, the benefits of Casey’s name might be finally beginning to fade.

“I think what Bobby has always had going for him is his dad’s legacy,” said Rep. Glenn “GT” Thompson (R-Pa.). “But I will say that I think the majority of voters in Pennsylvania are at an age where they don’t remember Bob Sr.”

Ally Mutnick contributed to this article.