Bob Dole, a former U.S. senator and the 1996 Republican presidential nominee, endorsed Trump in a statement released by the Trump campaign on May 6, 2016.
"The voters of our country have turned out in record numbers to support Mr. Trump," Dole said. "It is important that their votes be honored and it is time that we support the party's presumptive nominee."
He added that he would be in attendance at the Republican National Convention in Cleveland.
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Former U.S. Speaker of the House Newt Gingrich admonished Paul Ryan for his hesitancy in backing Donald Trump, saying that the current House speaker has a duty to "unify the party" behind the presumptive nominee.
"There is something wrong here when these people, who the Republican Party has done a lot for, let's be honest. The Bushes, the Romneys - the Republican Party has been pretty darn good to them, and they owe it to the party and the people who make up that party, beyond their own hope to be above it all," Gingrich told Fox News. "In the case of Paul Ryan he made a big mistake today and he needs to understand this."
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Chairman of the National Republican Committee Reince Priebus tweeted out his support of Trump shortly after Texas Sen. Ted Cruz's campaign folded. He declared Trump the "presumptive @GOP nominee" and said that "we all need to unite and focus on defeating" Democratic front-runner Hillary Clinton.
Priebus' history with Trump has been a fraught one, with the Republican nominee accusing the RNC chair of stacking the deck against him.
"I have committed to supporting the nominee chosen by Republican voters, and Donald Trump, the presumptive nominee, is now on the verge of clinching that nomination," McConnell said after Trump knocked out his rivals for the nomination.
Sen. Jeff Sessions, R-Alabama
Sen. Jeff Sessions, R-Alabama, became the first senator to endorse Trump in February. Sessions declared his support for the candidate at a rally just ahead of Alabama's primary and has backed him ever since.
Sessions, who is widely considered to be one of the most conservative members of Senate, said Trump was "committed to leading this country in an effective way" while acknowledging his flaws.
"Nobody is perfect," Sessions said during his endorsement. "We can't have everything, can we, Mr. Trump?"
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Sen. Kelly Ayotte, R-New Hampshire
New Hampshire Sen. Kelly Ayotte, who is facing a competitive race for her Senate seat this year, said she would support Trump as the nominee -- with the caveat that she would not officially endorse him.
"As she's said from the beginning, Kelly plans to support the nominee. As a candidate herself, she hasn't and isn't planning to endorse anyone this cycle," said Liz Johnson, communications director for Kelly for New Hampshire, in a statement.
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New Jersey Gov. Chris Christie
New Jersey Gov. Chris Christie was the first of Trump's former opponents to officially back the billionaire.
"I am proud to be here to endorse Donald Trump for president of the United States," Christie said at a Trump press conference in Fort Worth, Texas just days before 13 states hold primaries or caucuses on March 1. "The best person to beat Hillary Clinton in November on that stage last night is undoubtedly Donald Trump."
Christie has since stumped for Trump on the campaign trail.
"He is not a perfect man," Perry told CNN in a phone interview. "But what I do believe is that he loves this country and he will surround himself with capable, experienced people and he will listen to them."
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Maine Gov. Paul LePage
Maine Gov. Paul LePage, known for his own brash comments while in the state's executive mansion, endorsed Trump shortly after New Jersey Gov. Chris Christie joined the campaign as a surrogate.
"I'll be very honest -- originally I said I'd like to see a governor," LePage, who first endorsed Christie in the 2016 race, told New England radio host Howie Carr in an interview in February. "But unfortunately, the American people are not going a governor this year, so I'm gonna endorse Donald Trump."
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Media mogul Rupert Murdoch said in March that the Republican party "would be mad not to unify" behind Donald Trump if he became the GOP's presumptive nominee.
He's expressed admiration before for Trump's "winning strategy" on Twitter, saying that the New York real estate titan was "appealing across party lines."
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Republican megadonor and casino mogul Sheldon Adelson declared his support for Trump in a brief interview with the New York Times, where he said the New York real estate mogul "will be good for Israel."
"I'm a Republican, he's a Republican," said Adelson, a prominent pro-Israel activist. "He's our nominee. Whoever the nominee would turn out to be, any one of the 17 -- he was one of the 17. He won fair and square."