At least four top Republican senators say they’ve decided to skip July’s party nominating convention in Cleveland to campaign in their home states. Several others say they haven’t decided whether they’ll make the trip, and at least one will boycott the event if Donald Trump emerges as the Republicans’ presidential pick.

This year’s convention is shaping up as perhaps the party’s most pivotal – and contentious – in a generation, but Senate Republicans appear increasingly nervous about it. Many in the party are agonizing over the possible elevation of Trump or a contested convention that awards the nomination to someone else, either of which risks damaging the party and handing Senate control to Democrats.

Sen. John McCain, R-Ariz., the Republican Party's 2008 presidential nominee, says he'll be too busy running for his sixth Senate term to go to Cleveland for the convention.

House Speaker Paul Ryan even used a Tuesday night appearance on CNN to call on his colleagues in Congress to attend what he said “could be a great historical exercise.”

Sen. Susan Collins of Maine will attend the convention in Cleveland but will not seek to go as a delegate because she has access to the floor as a senator, according to Annie Clark, a spokeswoman for Collins. The senator attends such conventions without being a delegate to avoid taking up a delegate slot when there so many Maine Republicans who want them, Clark said in an email.

So far, most of the senators planning to skip the convention are being careful to avoid blaming Trump or the potential messiness directly. Sen. John McCain of Arizona, the party’s 2008 presidential nominee, said Tuesday that he’ll be too busy running for his sixth Senate term to go to Cleveland, with his own primary just weeks later.

“It’s not unusual for someone who’s up for re-election to be campaigning,” McCain said when asked whether he would rather avoid a potentially disordered convention. “I have a primary that’s on Aug. 30.”

McCain is facing what may be his toughest re-election fight and has clearly been uncomfortable with Trump’s success. Two others not attending – Mark Kirk of Illinois and Kelly Ayotte of New Hampshire – also face tough races against Democrats.

Even though those taking a pass on Cleveland say they just want to be on the campaign trail, staying away affords them other advantages. They might be able to avoid taking sides in what could be a protracted and bitter feud between Trump and Texas Sen. Ted Cruz, and perhaps distance themselves from any chaotic fallout – or an unpopular nominee.

But their absences also diminish the likelihood of any grand unity gesture emerging from the convention, if many of the party’s senior leaders and most well-known figures aren’t even in the room.

Alaska’s Lisa Murkowski noted the convention, which is scheduled to run from July 18 to 21, “is very shortly before my primary. So I’m going to be home with Alaskans.”

Asked whether fissures in the party over leading candidates Trump or Cruz contributed to her decision, she responded, “I won’t be there.”

Of course, it isn’t unprecedented for senators to skip the event. In 2008, when the Republican brand was damaged in the wake of the George W. Bush administration, a handful of vulnerable senators skipped the Republican convention in Minneapolis, including Collins, Pat Roberts of Kansas, Gordon Smith of Oregon, Elizabeth Dole of North Carolina and Ted Stevens of Alaska.

Trump’s bullying rhetoric and Cruz’s history of bucking his party’s Senate leaders could lengthen the list of lawmakers planning to sit out this year’s convention.

Sen. Jeff Flake of Arizona, who earlier endorsed Sen. Marco Rubio of Florida, said he’ll decide whether to go based on “whether it’s a Trump coronation or not. If it is, I see no reason to go.”

Sen. Lindsey Graham of South Carolina, an unsuccessful presidential contender who has said Trump’s policy pronouncements endanger stability in the Middle East and leave foreign leaders “dumbfounded,” said he’ll decide later whether to attend based on “how crazy it looks.”

Still, many other Republican senators are planning on going, including Majority Leader Mitch McConnell of Kentucky, Republican Whip John Cornyn of Texas and Rubio, who will arrive with some delegates won in primaries before his March 15 decision to suspend his presidential campaign.

At least one senator running for re-election will also be there: Rob Portman of Ohio, who would have trouble skipping such a gathering in his home state. He is, however, planning his own parallel convention, which could offer him some separation.

Some senior senators say they will go precisely because the wisdom of party elders may be sorely needed.

“I’ve never missed one,” Orrin Hatch of Utah said, adding he plans to go even if the convention gets messy. “Especially, because they’re going to have to have some people with brains, you know. I shouldn’t say that.”

Among those vulnerable senators on the ballot, a few are weighing making just brief trips to Cleveland. Sen. Roy Blunt of Missouri, who won his last election with 54 percent of the vote, said that if he goes it will only be for “a day or two.” He’s more focused on his Aug. 2 Republican primary, he said, and doesn’t plan to be a convention delegate, anyway.

Blunt said there’s some benefit to staying away for those up for re-election. If the convention is contested, he said, some will inevitably describe it as a “brokered” one, where powerful people in the party swoop in to help steer the outcome, rather than the delegates themselves.

“Certainly, elected officials who aren’t there are unlikely to be accused of being the people who are trying to broker the convention,” he said. Asked whether he’s concerned about being labeled a “broker,” he said, “I shouldn’t even be talking about this, because I haven’t decided what I’m going to do.”

Other senators up for re-election who are planning brief stays include Chuck Grassley of Iowa and Richard Burr of North Carolina. They each insist their past experiences have taught them that conventions suck up valuable time.

“I’ve gone to every convention since 1980, and every time I get done going I wonder why I went to the convention,” Grassley said. “And I think this time I’m going to talk to myself beforehand instead of afterwards. You waste a lot of time.”