Now, facing the reality of a Trump victory, they may take their money down-ballot to help Republicans in House and Senate races.
Art Pope, a former Marco Rubio donor and ally of the heavy-spending Koch brothers, said he will not support Trump and would spend elsewhere.
"Because I think Donald Trump's policies will harm America, I think it's more important to support conservative Republican candidates running for Congress and positions across America," Pope said Wednesday in an interview with CNN. "I would encourage everyone, starting with the voters, to pay very close attention to the down-ticket ballots, I think that's absolutely crucial."
Republican Party insiders are recalling the late stages of the failed 1996 Bob Dole bid -- where the Republican National Committee shifted its advertising in the closing weeks entirely to a down-ballot message of not giving Bill Clinton a "blank check" with the House and Senate.
"I think with either a Trump presidency or a Hillary Clinton presidency, I think (the) grass-roots will be very happy to have a Republican Congress to hold in check an authoritarian executive branch," Pope said.
In statements late Tuesday night after Cruz's departure from the race, anti-Trump super PACs vowed to continue operating.
"We continue to give voice to the belief of so many Republicans that Trump is not a conservative, does not represent the values of the Republican Party, cannot beat Hillary Clinton, and is simply unfit to be President of the United States," said Our Principles PAC Chairwoman Katie Packer.
"Never does not mean maybe," said NeverTrump senior adviser Rory Cooper in a statement. "Six out of ten Republican primary voters voted for someone other than Trump, and we will continue to identify ways to give them voice."
But Republican bundler and lobbyist Sam Geduldig had harsh words for the movement, and said he wants to see that money put to better use on the rest of the ballot.
"This never-Trump money that's wasted, I would rather that go straight into the campaigns of (Sens.) Ron Johnson or Kelly Ayotte," Geduldig said in an interview. "To me they're just flushing their money down the drain."
Down-ballot races offer a particularly appealing pitch to donors turned off by Trump.
Democrats only need to flip five seats in the Senate to win a majority, and have targeted nearly a dozen vulnerable incumbents, many in blue states, with challengers. Those challengers and the party have been eager to tie Trump to the senators, buoyed by polls that show Trump losing by double digits in a general election matchup with Hillary Clinton, like the latest CNN/ORC poll.
Republicans have a more comfortable majority in the House and have built a sizable advantage at the state level across the country in the past decade, but fear a Democratic wave election could erode those gains.
Mike Shields, a CNN delegate analyst, said in an interview that Trump's clinching of the nomination would likely hasten spending on the rest of the ballot, even if it hasn't begun yet.
"All the ingredients are there: storm clouds, conversations with donors I've had, you can tell that's where they're headed. They haven't written a check yet, but they're going to get there," said Shields, a former RNC chief of staff and president of the Congressional Leadership Fund and American Action Network. "Getting him to 1,237 will probably speed up the money coming to down-ticket, because everyone's waiting for that. And there's a significant amount of fatigue to be gotten through."
Geduldig agreed the writing was on the wall for donors.
"It doesn't take a bunch of rocket scientists to sit around and decide that that's our best method of attack," Geduldig said, referring to poor polling for Trump. "I'm pretty sure it's going to happen, but I've never heard anyone say yet they're going to be in charge of that effort."
Democrats are aware of the potential influx of cash into states and prepping for it.
"We totally recognize and take seriously that the dumpster fire that's shaping up at the top of the ballot could definitely direct some more resources toward these Senate races with an eye toward keeping the majority, and I think our recognition of that has actually been borne to bear with how well have done on fundraising," said Democratic Senatorial Campaign Committee press secretary Lauren Passalacqua in an interview.
Liberal groups are hopeful that Trump provides an opportunity for gain, said Gara LaMarche, president of the liberal donor network Democracy Alliance. But he also said the group is strategizing to prepare for more money at the state level.
"There's a lot of scuttlebutt and fear that the Kochs and people like that, who've always been focused on the states will cut their losses presidentially and put more money on the state, so I made the case to our donor weeks ago that that's all the more reason to be focused on the states," LaMarche told CNN.
Hope for Trump
There are signs Trump could mend fences with major donors who supported other candidates in the primary, though Shields notes he has a "lot of work" to do.
The real estate mogul has spent his primary eschewing political donations, saying he would self fund his campaign to not be beholden to lobbyists. While his campaign has raised some money outside of Trump's own pockets, it has not engaged in the coordinated fundraising effort that typifies modern presidential camapigns.
But Trump signaled Wednesday that he would fundraise
with the RNC for a general election campaign.
"I think it's most likely that I will, because you are talking about a billion dollars or a billion and a half dollars," he said when asked on "Fox and Friends" if he would begin fundraising. "I'm not looking for myself, I'm looking out for the party, so the party can compete in Senate races and House races. I want to raise money for the party."
Shields said it is a fallacy that all GOP donors are the same every year, saying it's up to each candidate to bring in their supporters to the fold.
And that could build good will between Trump and the party.
"Trump can solve this if he goes out and creates Trump donors that give money down ticket that he can claim credit for ... every one of these other candidates have done that," Shields said. "The down ticket people are grateful for that. So he can actually do a lot to help himself with the rest of the party if he can do that."