Speaker of the House Paul Ryan, R-Wisconsin, said May 5, 2016, that he's just "not ready" to support Trump for the presidency.

"I'm not there yet," said Ryan, who will chair the Republicans' convention in Cleveland this summer.

"Saying we're unified doesn't in and of itself unify us," he said of Trump's promise to gather the GOP together. "I think he has to do more to unify this party."

Former President George H.W. Bush and former President George W. Bush will not be backing the presumptive nominee, despite their record of endorsing the GOP's general election candidate in the past.

Spokesmen for the former commanders-in-chief said May 4, 2016, that they will not participate in or comment on Trump's presidential campaign.

Both endorsed Mitt Romney in 2012 and John McCain in 2008.

Jeb Bush, like his family members, will not be backing his party's presumptive nominee.

"In November, I will not vote for Donald Trump or Hillary Clinton, but I will support principled conservatives at the state and federal levels, just as I have done my entire life," the former GOP presidential candidate and former Florida governor wrote in a Facebook post. He slammed Trump as someone who was not a "consistent conservative" and as a candidate who has no respect for the Constitution.

Like his brother and father, Jeb Bush also endorsed Mitt Romney in 2012 and John McCain in 2008.

South Carolina Sen. Lindsey Graham, a former presidential candidate, is one of Trump's most outspoken critics in the Republican Party.

On CBS' "Face the Nation," Graham predicted that a Trump presidency would "lead to another 9/11" and charged the billionaire with starting a "civil war" within the GOP.

Graham was so vehemently anti-Trump that he had even endorsed Texas Sen. Ted Cruz, despite their strained Senate relationship.

Former Republican presidential nominee Mitt Romney has called Trump a "phony," a "fraud" and a candidate whose attitudes are "not the temperament of a stable, thoughtful leader."

In March, before Trump became the GOP's presumptive presidential nominee, Romney warned Trump's "imagination must not be married to real power."

Nebraska Sen. Ben Sasse called for a third-party challenger to Donald Trump May 5, 2016, writing a lengthy letter on Facebook to address the issue.

"Why shouldn't America draft an honest leader who will focus on 70-percent solutions for the next four years?" the GOP senator questioned. "You know...an adult?"

Sasse opposed Trump even before the billionaire became his party's presumptive nominee, having campaigned against him prior to Iowa's kickoff caucuses in February.

Former New Jersey Governor and moderate Republican Christine Todd Whitman has said she won't support Trump, and instead plans to vote for Hillary Clinton.

"You'll see a lot of Republicans do that," Whitman told the Star-Ledger's Tom Moran in February. "We don't want to, but I know I won't vote for Trump."

Whitman also served as EPA administrator in George W. Bush's administration.

Bill Kristol, editor of the Weekly Standard, has been trying to recruit a candidate for a third-party run against Trump. He's made no secret of his efforts, and he met with 2012 GOP Mitt Romney in early May to try to talk about the possibility of a 2016 bid, the Washington Post reported.

"He came pretty close to being elected president, so I thought he may consider doing it, especially since he has been very forthright in explaining why Donald Trump and Hillary Clinton should not be president of the United States," Kristol told the Post.

Kristol says that Trump lacks the character to be president.

Former Pennsylvania Gov. and former Homeland Security Secretary Tom Ridge has called Trump "an embarrassment" to the Republican party and says that he will never vote for Trump.

"I think Donald Trump is about celebrities, he's about publicity. He's not about bringing a serious mindset toward trying to address the wide range of security challenges in this country," Ridge said at a New Hampshire campaign event for Jeb Bush in December.

Former Oklahoma Rep. J.C. Watts told the Wall Street Journal in February, "It's going to be a tremendous setback for the party if he wins," and he added, "All these guys who are beating him up now -- if he asks them to be his running mate, they'll jump in in a New York minute."

Watts had backed Rand Paul before Paul dropped out of the race.

Former Florida Sen. and former RNC chairman Mel Martinez rejects the idea of voting for Trump.

"If there is any, any, any other choice, a living, breathing person with a pulse, I would be there," he told the Wall Street Journal in February.

Sen. Marco Rubio said on May 9 that he isn't interested in being Donald Trump's running mate because he still has reservations and concerns with the presumptive GOP nominee's campaign and policies.

"He will be best served by a running mate and by surrogates who fully embrace his campaign. As such, I have never sought, will not seek and do not want to be considered for Vice President," the Florida senator and former GOP presidential candidate wrote in a post on Facebook.

Sen. Mike Lee, who had endorsed his friend Ted Cruz, has reservations about presumptive nominee Donald Trump.

"I have some concerns with him. He scares me to death; so does Hillary Clinton," Lee said on May 11, according to the Washington Examiner. There is no easy choice right now."

"I'll make the decision as best I can," Lee added, "but I'm not there yet."