The major Never Trump groups are all funneling their cash into Indiana as conservative donors increasingly view the state as essential to stopping the Republican presidential front-runner.

"Indiana is crucial," said Doug Sachtleben, spokesman for one of the best-funded anti-Trump super-PACs, Club for Growth Action (CFGA), which says it has already spent at least $1.7 million, mostly on statewide TV advertising, opposing Trump in Indiana.

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"We think Ted CruzRafael (Ted) Edward CruzPoll shows competitive matchup if O’Rourke ran for Senate again Democrats veer left as Trump cements hold on Republicans O’Rourke heading to Wisconsin amid 2020 speculation MORE must win and will win Indiana," Sachtleben added. Club for Growth's Indiana ad, titled "Math," says that a vote for Kasich helps Trump because it divides the Republican field.

Trump's rivals are hoping to use the delegate process against the front-runner. If they can strategically keep Trump from winning 1,237 delegates in the state contests, they can force a contested Republican National Convention. At this point, that is just about the only chance either has of winning the nomination.

"The Kasich team has essentially endorsed the message of the CFGA ad in Indiana, with the understanding that to stop Trump in Indiana, voters should vote Cruz," Sachtleben said, referring to the Kasich campaign's decision to withdraw from competing in Indiana, effectively ceding the state to Cruz.

Three other major Republican-aligned anti-Trump groups are homing in on Indiana to stop Trump's momentum, which has accelerated after his big wins in New York last week and throughout the Northeast on Tuesday. The Indiana strategy is similar to the approach taken by anti-Trump forces in the Wisconsin primary, where Trump lost badly to Cruz.

Our Principles PAC — an anti-Trump group funded by GOP mega-donors including billionaire hedge fund manager Paul Singer and the Ricketts family, which owns the Chicago Cubs baseball team — is spending at least $1 million in Indiana opposing Trump, said the group's chairwoman, Katie Packer, though she declined to disclose the exact amount of spending.

"We have got a full-court press going there," Packer said on Wednesday. "It's a pretty comprehensive TV buy that is in place statewide. We have got a full mail program, digital — it's really the scope of what we did in Wisconsin."

"We certainly see Indiana as crucial for Trump. If he doesn't win Indiana, he can't get to 1,237," Packer added. "If he does win Indiana, then it all comes down to California.

"No question about it, it makes our job a lot harder if [Trump] wins Indiana."

Our Principles PAC, which has already spent more than $16 million opposing Trump in primary and caucus states, released a memo shortly before Trump's clean sweep of the Northeast primaries on Tuesday. The memo accurately predicted Trump's overwhelming victories but insisted that the billionaire could still be denied the 1,237 delegates.

Indiana holds a winner-take-most contest. The statewide winner will receive 30 delegates, and 27 more will be awarded through winner-take-all contests in each congressional district. 

Trump sits at 987 delegates, according to the Associated Press delegate tracker, and needs fewer than 50 percent of the remaining delegates to lock up the nomination. Both Cruz and Kasich are mathematically unable to reach that mark without a contested convention.

A Trump victory in Indiana could leave him needing as little as 43 percent of the remaining delegates, and a total loss would leave him needing 56 percent.  

While Trump is seen as a lock for New Jersey’s 51 winner-take-all delegates, the remaining contests present few options for Trump to stockpile delegates outside of Indiana and California.

The fervent hope now within the anti-Trump movement is that the billionaire's delegate accumulation can be slowed in Indiana and then ultimately stopped short in the June 7 California primary.

If Cruz and the anti-Trump forces can keep Trump from the 1,237 mark and convince unbound delegates not to push him over that threshold on the convention’s first ballot, it would reset the contest.

Cruz has bested Trump in winning the loyalties of delegates — including those bound to Trump — and after the first ballot, the lion’s share of them would be free to vote their conscience.

Packer said that Our Principles PAC is targeting three states in particular: Indiana, Nebraska and California. 

So important is Indiana to the Never Trump movement that the nonprofit conservative group, American Future Fund, which pulled back its spending after spending spent millions against Trump leading up to Super Tuesday in March, is likely to rejoin the fray.

While the amount American Future Fund will spend remains unclear, a source familiar with its plans told The Hill the group will be "active" against Trump in Indiana.

The nonprofit has a history of aggressive spending and has proven it has deep pockets. It has past ties to the powerful conservative donor network helmed by billionaire industrialists Charles and David Koch, though the source of funding for the group's anti-Trump drive and its remaining funds are unknown.

Also joining the Indiana assault against Trump is the digital-focused group within the anti-Trump movement, the Never Trump PAC. The group, which is run by experienced GOP operatives,  has a smaller budget than the other groups. But it plans to spend tens of thousands of dollars on targeted digital communications designed to undermine Trump's chances in Indiana, said spokesman Rory Cooper.

Never Trump PAC has been running digital ads in Indiana since Monday. Cooper said the state is "extremely important ... not just from a math perspective but from a narrative perspective."

The ads are a mix of attacks against Trump and messages designed to persuade voters who don't like Trump to act strategically and vote for Cruz rather than Kasich to stop the billionaire, Cooper said.

"We think Indiana is important because lots of members of the media want to call this race," Cooper added. "They think the Never Trump movement is dead. ...They want to wrap this up and perception is often reality in political races."

"Ted Cruz has the opportunity to reset this race" in Indiana, Cooper said.

"It gets harder, much, much harder, if Ted Cruz is not successful in Indiana."