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Cliff Owen, Associated Press
Democratic presidential candidate, Sen. Bernie Sanders, I-Vt. speaks at a news conference in Washington, Sunday, May 1, 2016.

TERRE HAUTE, Ind. — The 2016 presidential campaign rumbled into Indiana Sunday focused on Tuesday's critical primary, even as front-runners Hillary Clinton and Donald Trump itched to fully engage in the one-on-one battle they cast as inevitable.

But the underdogs in both parties made clear they had no plans to exit the race, at least until the Indiana results come in — and perhaps longer.

"We're going the distance," Trump rival Ted Cruz said on ABC's "This Week," arguing that Trump won't be able to get the majority of delegates required to clinch the nomination. "We're going into Cleveland, and it will be a contested convention."

On the Democratic side, Bernie Sanders insisted that his path to the nomination depends on the unlikely prospect of flipping superdelegates who are now committed to Clinton. Superdelegates can vote for either candidate. The former secretary of state is still 91 percent of the way to the nomination, according to The Associated Press. She is 218 delegates away from winning the 2,383 need to clinch the nomination.

"We have an uphill climb, no question about it," he said, before hopping a plane to Indiana to continue the contest.

And so the stalemate between the front-runners and their struggling rivals continued.

The frustration was dramatic on the Republican side. Campaigning in Terre Haute, Indiana, Trump again reiterated that he believes the GOP race is over, something he's been saying for days even though he hasn't secured the 1,237 delegates required to win the nomination. He groused that Cruz and Ohio Gov. John Kasich should still get out because they are forcing him into "wasting time" that he could otherwise spend raising "money for the Senate races."

That overt offer of fundraising is new for Trump, incentive for Republican leaders to help push Cruz and Kasich out of the race. Senior adviser Paul Manafort further telegraphed the message Sunday on CBS' "Face the Nation," saying that Trump is looking to strengthen ties to "leaders of the Republican Party and various committees to help raise money for them."

Clinton, in Indianapolis, did not bother mentioning Sanders' name. Instead, she criticized Trump for embracing GOP economic policies that have left everyday workers behind. And she took aim at both Trump and Cruz for wanting to "slash taxes on the wealthy" and for using "dangerous" rhetoric about Muslims.

Cruz wasn't surrendering to the delegate math, even after a tough week in which former House Speaker John Boehner called him "Lucifer in the flesh" and "a miserable son of a bitch." Cruz pointed out on several political talk shows that Indiana Gov. Mike Pence and former California Gov. Pete Wilson have endorsed him and that Trump can't get a majority of Republicans to back him.

The Cruz campaign has put an emphasis on Indiana and a loss here could be perceived as crippling to his campaign, which is perhaps why the candidate himself has shifted to talking about competing in next month's California primary and beyond.

Trump dominated the talk show conversation Sunday. On ABC, the first question posed to former CIA director and defense secretary Robert Gates was about what a Trump candidacy would mean for the nation's national security.

"I think based on the speech, you'd have somebody who doesn't understand the difference between a business negotiation and a negotiation with sovereign powers," Gates, who has worked for both Republican and Democratic presidents, replied. "He doesn't understand that there's a give-and-take in international relations that is different than in the business community."

On CBS, Sen. Lindsey Graham, who has endorsed Cruz even though he has said he loathes the Texas senator, said Trump's foreign policy amounts to "isolationism. It will lead to another 9/11."

Graham added on CBS: "Hillary Clinton is an incredibly flawed candidate, but she will mop the floor with Donald Trump."

Meanwhile, Sanders was facing a new round of questions about why he was even still running.

"It's difficult, it's not impossible," Sanders said on CBS's "Face the Nation" of his increasingly bleak challenge to Clinton.

Kellman reported from Washington. Associated Press writers Lisa Lerer in Washington and Brian Slodysko in Indianapolis contributed to this report.

On Twitter, follow Jonathan Lemire at twitter.com/JonLemire and Laurie Kellman at www.twitter.com/APLaurieKellman

This story has been corrected to reflect that Graham spoke on CBS, not CNN.