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 Cruz Rafael (Ted) Edward CruzSenate Republicans face tough decision on replacing Ginsburg Cruz: Trump should nominate a Supreme Court justice next week Renewed focus on Trump's Supreme Court list after Ginsburg's death 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.