Even as Donald Trump and Republican Party bosses diligently work Capitol Hill in hopes of bringing the party together after a fractious presidential primary, convention planners could still be looking at a block of empty seats for the July convention.

A growing roster of senior GOP figures – from governors to senators to, most notably, nearly every living GOP presidential nominee – is vowing to skip the convention in Cleveland, despite the candidate starting to win over the rank-and-file.

In an unconventional election season where Trump has capitalized on an anti-establishment fervor, the case can be made that Trump does not need the blessing of party elders, or their attendance.

“Trump is a master entertainer and more than likely going to put together a convention program that attempts to highlight his strengths and sideline some of the major absences,” Republican strategist Ron Bonjean told FoxNews.com.

Still, since wrapping up the nomination, Trump and his surrogates have been regularly meeting with Hill Republicans, showing at least an effort to pursue party unity – a message that high-profile absences in Cleveland could undercut.

Trump hit the unity theme again Sunday night, as he responded to the latest prediction that an independent candidate would soon enter the race. On Twitter, Trump warned, “if the GOP can't control their own, then they are not a party.”

Yet Trump’s contemporaries will be nowhere near Cleveland.

Of all the living Republican presidential nominees and former presidents, only Bob Dole is expected to attend – and even then, only “briefly,” for the purpose of catching a luncheon hosted by his law firm, a source told Fox News earlier this month.

Former Presidents George H.W. Bush and George W. Bush have said they will not attend, as have 2008 nominee John McCain and 2012 nominee Mitt Romney. Former 2016 White House candidate Jeb Bush also is expected to skip.

Of them, Romney is working most actively against Trump, having delivered a major address attacking his candidacy and frequently sparring with the now-presumptive nominee on Twitter. He also reportedly has been the focus of efforts to recruit an independent candidate, though so far to no avail.

Others claim to be skipping in order to focus on their own election battles – some of those potentially made more challenging by Trump’s primary success.

McCain seemingly counts himself among that group. The Arizona senator is facing a tough re-election fight in a state with a heavy Hispanic population, and has said Trump complicates his race.

New Hampshire Sen. Kelly Ayotte also has said she will not be attending the convention, citing a tough re-election battle.

"Unlikely," Ayotte told CNN. "I've got a lot of work to do in New Hampshire, I have my own re-election and I'm going to be focusing on my voters in New Hampshire."

Other senators who do not plan to be in Cleveland include: Wisconsin’s Ron Johnson, Alaska’s Lisa Murkowski, Kansas’ Jerry Moran, Missouri’s Roy Blunt, and Illinois’ Mark Kirk, according to McClatchyDC. Sen. Richard Burr, R-N.C. told CNN "I’m more valuable outside of Cleveland than inside of Cleveland," but his spokeswoman later said his schedule is still being finalized.

FoxNews.com reached out as well to Republican governors for an attendance tally.

Most of those RSVPs remain outstanding, but representatives for Nevada Gov. Brian Sandoval and Wyoming Gov. Matthew Mead told FoxNews.com they would not be in Cleveland. Mead’s spokesman cited a busy summer as the reason for the governor not making it.

No-shows could be more common for lawmakers in the House, where a faction remains skeptical of Trump’s candidacy. Rep. Mick Mulvaney, R-S.C., a co-founder of the House Freedom Caucus, told The Hill about 10 other conservatives are planning not to attend.

Many lawmakers and their staff remain tight-lipped about whether they’re attending, and their rationale.

Asked for comment on whether Sen. Jeff Flake, R-Ariz., would show up, a representative for the senator sent FoxNews.com a Boston Globe article in which Flake is quoted as saying, “I’ve got other things to do.” Meanwhile, a rep for South Carolina Gov. Nikki Haley linked to a YouTube video in which the governor said she was undecided on whether to go.

This may be because Republicans are still evaluating how to deal with Trump’s victory.

House Speaker Paul Ryan, for one, is rumored to be close to endorsing the billionaire, though he hasn’t yet. Now that Trump has reached the necessary 1,237 delegates to clinch the nomination, according to the latest AP tally, more top GOP lawmakers could feel pressured to step in line.

Marco Rubio, once an outspoken Trump rival who sold #NeverTrump merchandise on his website, said Sunday he will get behind the presumptive nominee.

“I want to be helpful,” the Florida senator said on CNN's "State of the Union."

The big unknown is whether Texas Sen. Ted Cruz, who suspended his campaign in early May, will similarly get on board.

While the absence of major figures at the convention could damage the party’s ability to present a united front against likely Democratic nominee Hillary Clinton, some argue that Trump will be able to manage the dissension in the ranks.

“The absences won’t hurt Trump’s ability to unite the party,” Bonjean said, “but he must keep his focus now on Hillary Clinton and avoid getting into fights with other Republicans that may not agree with his pending nomination.”

It is not unheard of for Republicans, especially those in tight re-election races, to skip the national convention when political waters look choppy.

In 2008, several top Republicans chose to skip. Some were due to urgent state issues -- like then-California Gov. Arnold Schwarzenegger grappling with a budget stalemate or Louisiana Gov. Bobby Jindal dealing with Hurricane Gustav. Gustav even forced then-President Bush not to attend, making him the first sitting president since Lyndon Johnson not to attend his party’s convention. Instead, he delivered his address by satellite hookup from the White House.

Sens. Pat Roberts, of Kansas; Susan Collins, of Maine; Gordon Smith, of Oregon; and Elizabeth Dole, of North Carolina, also stayed home from the GOP convention in order to campaign. At that time, Bush’s approval ratings were hovering around 30 percent.

This was a departure, however, from the 2000 convention where various interest groups in the party all came together behind their candidate with a single-minded determination to take back the White House. It didn't hurt that Bush was leading in the polls and had seen his last primary challengers fade away months before.

At that convention, there were no reports of prominent Republicans choosing not to attend.

Adam Shaw is a Politics Reporter and occasional Opinion writer for FoxNews.com. He can be reached here or on Twitter: @AdamShawNY.