Sen. John McCain, R-Arizona, who was the GOP's presidential nominee in 2008, is facing a tight reelection race and has decided to opt out of the convention.

Donald Trump insulted McCain last summer when he questioned his reputation as a war hero because he was captured in Vietnam.

The 2012 Republican presidential nominee, Mitt Romney, has not been shy about his opposition to Trump. Romney, who also served as governor of Massachusetts, decided in early 2015 not to run for president again.

Romney has called a "phony" and a "fraud" and unfit to serve as president.

Ohio Gov. John Kasich, the sitting governor of the state, told the Washington Post he has no plans to speak at the convention. As the home-state governor, he'll be in Cleveland and attending events, but will not be seen with Trump. In June he had told Fox News that he didn't know whether he could support Trump for president. "Right now, the divisiveness, the division, the name calling -- it just doesn't go down well with me," he said.

Republican presidential candidate John Kasich speaks to members of the media before speaking at the California GOP convention in Burlingame, California, April 29, 2016.

Former Florida Gov. Jeb Bush recently reiterated that he can't vote for either Donald Trump or presumptive Democratic nominee Hillary Clinton, saying that "conservatism is temporarily dead" while Trump is the party's standard-bearer.

Bush, who dropped out of the GOP presidential race earlier this year, won't be attending the convention, he said, but added that he'll "probably watch, sure."

Sen. Marco Rubio, R-Florida, initially said in May that he would be willing to attend and speak at the convention, but changed course when he decided to run for reelection late last month.

"Marco had planned to go to the convention before he decided to seek reelection," his spokeswoman told the Tampa Bay Times. "Since Marco got into the race late, he will be in Florida campaigning and meeting with voters instead of going to Ohio."

Sen. Lindsey Graham, R-South Carolina, is planning to steer clear of the convention.

While he has warmed up to Donald Trump at times, he has also warned that a Trump administration could lead to another 9/11.

Sen. Mark Kirk, R-Illinois, one of the most vulnerable senators up for reelection, has said that he cannot support Trump for president, and has called him "too bigoted and racist" to be commander in chief.

Sen. Kelly Ayotte, R-New Hampshire, also faces a tough reelection race and has said while she plans to support the GOP presidential nominee, she doesn't plan to endorse any candidate.

Sen. Roy Blunt, R-Missouri, who's up for reelection, said he plans to support the nominee, but is not attending the convention.

It will be the first convention in 16 years that Blunt has decided to skip.

Sen. Lisa Murkowski, R-Alaska, is up for reelection and doesn't plan to attend the convention.

South Carolina Gov. Nikki Haley, who is perennially touted as a possible national candidate and placed on potential VP lists, "has not been asked to speak at the convention and has no plans to."

Ben Sasse, the Nebraska senator who's served as a sort of frontman for the Never Trump movement, has made it clear he will not be at the convention either.

New York Rep. Elise Stefanik, a young first-term congresswoman and former Paul Ryan staffer from New York who helped write the 2012 GOP platform.

A spokesman told Politico she "will be in her district working for her constituents and not attending the convention."