(CNN) Donald Trump is barreling toward a collision with fellow Republicans, upending recent progress in uniting the party -- and even threatening to go it alone if GOP leaders don't fall in line.

The presumptive GOP nominee on Wednesday went so far as to tell its elites that he did not need them at all.

And leading Republican fixtures, who just a month ago had been inching slowly yet unmistakably toward the Trump banner, are now pacing the other way, revoking endorsements and even considering ways to throw him overboard at the Republican National Convention. Trump's unwillingness to wage a traditional campaign is inviting a new round of handwringing from GOP elders, whose worries have only intensified after much-criticized Trump's reaction to the massacre at an Orlando nightclub this past weekend.

And the hostility is mutual. At the Fox Theater in Atlanta on Wednesday, Trump expressed himself more candidly than he has ever since becoming the presumptive nominee in early May.

"We have to have our Republicans either stick together, or let me just do it by myself. I'll do very well," Trump told the crowd. "A lot of people thought I should do that anyway, but I'll just do it very nicely by myself."

In the six weeks since he cleared the Republican field, he's done just the opposite: He inked a fundraising deal that tethered himself to the Republican National Committee and made him reliant on its donors to bail him out of his fundraising deficit. He courted prominent conservatives like Paul Ryan and leading financiers like Sheldon Adelson -- and landed them. And a bevy of Republicans, from Capitol Hill insurgents to almost every presidential hopeful that he vanquished in the primary, came to his side, pitching him as a less-than-ideal candidate but one that could win and save the Supreme Court.

That's over.

Some Republicans are distancing themselves completely, like Maryland Gov. Larry Hogan, who said Wednesday he wouldn't back Trump. Others, like Mark Kirk of Illinois, backtracked on their initial blessings.

Most Republicans are merely holding out on one hope or another. Some are hoping to educate him, praying that he will recalibrate and that he can be convinced to change his ways. His reluctance to immediately back away from inflammatory comments about a Hispanic federal judge last week convinced few that this path is plausible.

"I don't think you can go around the country saying that we need to keep people out of the country based solely on their religious belief and expect to be president of the United States," Senator Lamar Alexander, who maintained that Trump was not yet the Republican nominee because the convention had not happened, said Wednesday. "But hopefully he'll change that."

Senator Jeff Sessions of Alabama, the first upper chamber Republican to endorse Trump, on Wednesday told CNN's Erin Burnett on "Erin Burnett OutFront" that he thinks Trump's endorsements from GOP leadership are "secure."

"I think he's going to have a strong campaign, and I think we're going to see more and more people move toward him," Sessions said.

A second group hopes that Trump might ease their concerns with a credible, conservative vice-presidential choice, who could guide the freshman presidential candidate on the campaign trail.

A third group, popular with an increasingly large set of Republican donors, is to funnel their time into House and Senate races and weather the fallout of Trump's fate. Senior party officials are worried that would essentially leave Trump out to dry, exacerbating a fundraising gap with Hillary Clinton that is already likely to eclipse $500 million.

"I'm running my own independent campaign back home -- have from the start," said Sen. Rob Portman, an endangered incumbent from Ohio who sits on a massive warchest. "And we'll continue to do that."

And yet a fourth group has emerged in recent days, with the most provocative proposal: to dump Trump at the Republican Convention should he not improve over the next two or three weeks. Given Trump's delegate haul, that would require a major change in RNC rules to trigger an open convention.

One name is emerging as the saving grace: Wisconsin Gov. Scott Walker, who has privately told friends he is "intrigued" by the possibility of allowing his name to be put in the ring at the GOP convention as a challenge to Trump, two sources with direct knowledge say.

Walker, who has said he would support the Republican nominee but last week said that Trump is "not yet" that person, denied he is even considering the possibility in a statement.

"Let me be clear: I am focused entirely on being Governor," he said in a statement from his office. "If there's any campaign in the future, it's going to be running for re-election in 2018, which is a decision that we'll make in the months ahead following the next state budget."

The anti-Trump sources, however, who asked to remain anonymous, say that while Walker has publicly denied any interest, the possibility was not dismissed when discussed with him privately. And they say it is a plus that he left the race early and was never defeated in the primaries.

"It's clearly planted in his mind," one source said. "How real this all becomes depends on a lot of things."

Previous efforts to find a Trump alternative have fizzled. These anti-Trump forces openly admit a Trump ouster is a long shot, and that they are looking for a white knight who shows any interest at all.

But their newest efforts -- fanned by Trump's drop in the polls and his recent controversies over the judge and his response to Orlando -- depend largely on whether, as one source put it, "the grassroots starts to recognize what a disaster this would be for the party."

This last-gasp scenario, its proponents argue, is not possible without the implicit support of Sen. Ted Cruz, who holds 563 delegates, according to the latest CNN estimate. They say that Cruz is not going to lead any kind of insurgency against Trump, but it's within the realm of possibility that -- given enough public support of this effort -- he could sign on.

They also point out there's plenty of big funders still sitting on the sidelines. And so these anti-Trump forces are now trying to flip big-name Trump endorsers.

"We need one prominent Trump supporter to speak out and renege," one source said. "Then the dam could break."