Having overcome a multimillion-dollar "Never Trump" campaign aimed at blocking him from the Republican nomination, he's now benefiting from a wave of GOP donors, party leaders, voters and conservative groups that are uniting under a new banner: "Never Hillary."
"Nothing unites Republicans better than a Clinton," says Scott Reed, a political strategist for the U.S. Chamber of Commerce who has advised previous GOP campaigns. While Reed says there remain "many unknowns" about Trump, he adds that "the knowns about Hillary are very powerful motivators to Republicans."
Thanks to Republicans' deep disdain for the likely Democratic nominee, Trump is piling up those kinds of lukewarm GOP endorsements.
Florida Sen. Marco Rubio, who called Trump a dangerous "con artist" during his own failed presidential campaign, now says he's willing to get involved in the general election to stop Clinton.
"If you can live with a Clinton presidency for 4 years, that's your right," Rubio wrote on Twitter Friday. "I can't and will do what I can to prevent it."
Later in the day he reiterated on Twitter that his assistance should not be viewed as pro-Trump. "I said I would be 'honored' to help party beat Hillary," he wrote.
"Never Hillary" graced the subject line of a new Republican National Committee fundraising email that had nary a mention of Trump. Super PACs advised by Trump-skeptic Karl Rove are using the hashtag "NeverHillary" on Twitter to promote online videos about her perceived scandals — even as Rove says the groups aren't likely to spend money boosting Trump.
Last week when the National Rifle Association endorsed Trump, the announcement came without much of a sales pitch for him. But it did include a blunt message for the 5 million members about Clinton.
Noting the heated GOP primary campaign, Chris Cox, the NRA's chief lobbyist, said at the organization's convention last week, "Were there differences between candidates for the nomination? Of course. Are there valid arguments in favor or some over others? Sure. Will any of it matter if Hillary Clinton wins in November? Not one bit."
For the NRA and other Republican-leaning groups, Clinton has become a reason to look past Trump's spotty record on conservative issues.
On guns, for example, Trump previously backed an assault weapons ban. He's since backed away from that, which appears to be good enough compared to Clinton's calls for tougher gun control laws.
"If she could, Hillary would ban every gun, destroy every magazine, run an entire national security industry right into the ground and put your name on a government registration list," NRA chief Wayne LaPierre told the crowd at the gathering in Louisville, Kentucky.
Likewise, Clinton has been an entry point for big donors once not thrilled with — or even downright hostile to — Trump.
Billionaire Minnesota broadcasting executive Stanley Hubbard helped pay for the Never Trump campaign, but says he's willing to give money to the GOP nominee to stop Clinton.
Trump has unclear policies on some of the issues most important to conservative donors. Even so, Foster Friess, who backed Rick Santorum in his last two presidential campaigns, said he has made a donation to Trump because "the choice is stark."
In an email, he contrasted Clinton's possible Supreme Court picks with Trump's, as well as their approaches to economic and immigration policies.
Sheldon Adelson, a billionaire casino executive whose top issue is the protection of Israel, urged reluctant Republican Jews to unite behind Trump.
"Like many of you, I do not agree with him on every issue. However, I will not sit idly by and let Hillary Clinton become the next president," he wrote in an email to fellow board members of the Republican Jewish Coalition.
Clinton has served as a call-to-arms for some of the top fundraisers for Trump's vanquished rivals, helping him quickly assemble an experienced finance team from scratch.
Some Republican voters, too, are finding that unease with Clinton is a good enough reason to back Trump.
Margaret Lee, a 66-year-old from Clayton, North Carolina, said that while the former reality TV star may not have been her first choice, she'll vote for anybody but Clinton.
"Hillary Clinton is not being held accountable," Lee said of Clinton's use of private emails as secretary of state. "The fact that she's going to be the Democratic nominee having this hanging over her head, I just can't understand that."
In Pennsylvania, Lori Clifton said she's deeply frustrated by the prospect of an election face-off between Trump and Clinton. Clifton, a 51-year-old from the Philadelphia suburb of Doylestown, isn't a Trump fan. But as a reliable Republican voter in presidential elections, she said, "What choice do I have? I really don't trust Hillary Clinton."
Alison Scott, a 36-year-old from Apex, North Carolina, also has concerns about Trump's demeanor, saying he often "doesn't seem very presidential." But with Clinton as the only alternative, she said her decision is simple.
"If I had to pick one of those," Scott said, "I'd vote for Trump."