Donald Trump became the Republican party's standard-bearer Tuesday night, after he swept the Indiana primary, knocking his one remaining credible rival out of the race and earning the title "presumptive nominee" from the Republican party's chairman, Reince Preibus.

There should be no stopping him now. But Republican operatives in the "stop Trump" movement say their war with the man who will be the Republican nominee for president is far from over.

One of the biggest anti-Trump groups, fittingly named Never Trump PAC, said Tuesday night that it's sticking with its mission, despite the overwhelming reality blaring across television screens that the battle is already lost.

"Obviously Trump's victory in Indiana makes the road ahead more challenging," the group said in a statement Tuesday evening. "We will continue to to seek opportunities to oppose his nomination and to draw a clear line between him and the values of the conservative cause."

"Never does not mean maybe," the group added.

Never Trump PAC's main focus now, it said, is to continue to wage war against their party's nominee in order to protect Republican candidates down the ballot in 2016, as the party tries to maintain its control over Congress.

The group includes several prominent Republican staffers like Rory Cooper, who worked in the George W. Bush White House and more recently led the communications shop for former House Majority Leader Eric Cantor.

"We must separate Trump's demagoguery from the conservative cause and protect GOP down-ballot candidates," Cooper said in a tweet on Tuesday night.

Never Trump also circulated a petition Tuesday night, asking conservatives to sign on vowing to never cast a vote for the presumptive GOP nominee. The petition currently has almost 30,000 signatures.

"I guess [Trump's] not quite at the magic number yet, anything could hypothetically happen. (...) I could also win the lottery today or get a pet unicorn."

Conservatives Against Trump, another group vowing to oppose the GOP's presumptive nominee, said Tuesday night that while it is "dismayed" by Trump's victory, it is not "deterred."

"We believe a Trump nomination is not inevitable despite tonight's result," Conservatives Against Trump spokesperson Deborah DeMoss Fonseca said in a statement. "We believe that an open and fair convention can produce a viable Republican candidate. As a coalition, we will continue to pursue this goal."

The group did not specify how it plans to stop Trump at this stage, but called on delegates not to "sign off on" his nomination at the convention in July.

But in many cases, the delegates' hands are tied. A majority of the delegates in states that have already voted will be required under party and state rules to support him on the first ballot. If Trump clinches the 1,237 delegates required to win the nomination (an increasingly likely possibility) before the convention, and without a viable alternative, a second ballot is highly unlikely.

"Trump is a walking, talking, in-kind donation to Hillary Clinton and the Democratic Party," Fonseca warned, adding that the frontrunner has shown "remarkable weakness" in key swing states like Colorado, Iowa, Ohio, Missouri, Virginia and Wisconsin.

Make America Awesome, another anti-Trump PAC that caught headlines for its ad featuring a nude photo of Trump's wife Melania earlier this year, was less clear about its future after disappointing results on Tuesday.

Founder Liz Mair emailed supporters Wednesday morning acknowledging that "it now looks overwhelmingly likely that the combined efforts of the #nevertrump movement will have failed." Mair said that she will send out a survey to the group's email list this week, asking them how they think the group should move forward. Options include spending money to try to sway delegates to oppose Trump at the Republican convention, working to get a conservative third-party candidate into the race, supporting down-ballot candidates who would block any executive overreach by either Clinton or Trump, and just throwing in the towel altogether.

Mair said in separate statement on Tuesday that she and co-founder Rick Wilson, a prominent Republican consultant, tried to warn party donors as early as October of 2015 that Trump would win the party's nomination "unless something dramatic and unconventional is done." But the group's efforts to stop him were hobbled by a lack of donor interest in going after the reality-TV star, at a time when political pundits and prognosticators assumed his candidacy would be short-lived.

Mair said that her group and other anti-Trump efforts were "severely hampered by a lack of interest and testicular fortitude on the part of a lot of people who could easily have changed the equation in this entire race with some earlier and robust action — or indeed, in many cases, any action whatsoever."

"For too many Americans, the choice between Hillary Clinton and Donald Trump feels like a choice between being shot in the head, and being shot in the head, with perhaps the only 'third way' being a vote for the Libertarian Party candidate or a write-in, just to ease the conscience," Mair said.

In the meantime, Mair asked supporters to call every one they know in the nine states where Republicans have yet to cast their ballots and encourage them to support John Kasich, in the hopes that he can prevent Trump from earning 1,237 delegates before the convention.

"In addition, if he sticks around, at least people showing up at his events will get free hugs, instead of sucker punches and/or a festering mob mentality, and that has to be worth something in this environment," she added.

But Kasich advisers announced just a few hours later that he will end his campaign on Wednesday evening. The Ohio governor had assured everyone on Tuesday night that he was still running for president, despite sitting in fourth place in what was a three-man race.

"Tonight's results are not going to alter Gov. Kasich's campaign plans," his team said in a statement. "Our strategy has been and continues to be one that involves winning the nomination at an open convention."

But by Wednesday morning that plan had changed. Kasich cancelled a press conference in Virginia and said he would speak in his home state of Ohio at 5pm instead. Long after it became mathematically impossible for Kasich to win before the convention, and less than 24 hours after Cruz decided to throw in the towel, Kasich will end his campaign.

That leaves anti-Trump forces with no one to turn to in the presidential race. But some conservatives remain in denial.

Sarah Rumpf, a longtime political operative and vocal member of the Stop Trump movement, tried to sound hopeful on Wednesday morning.

"I guess [Trump's] not quite at the magic number yet, anything could hypothetically happen," she said with resignation. But, she added, "I could also win the lottery today or get a pet unicorn."

Rumpf is refusing to vote for Clinton and said she would probably support a third party, but vote for the Republican candidates in down-ballot races. But she, like many other conservatives, isn't exactly thrilled about the situation their party is in.

"Republicans are nursing a metaphorical, and in some cases literal, hangover this morning," she said.

Follow Olivia Becker and Sarah Mimms on Twitter: @OliviaLBecker and @SarahMMimms