INDIANAPOLIS (AP) — Once dismissed as a fringe contender, businessman Donald Trump now is all but certain to lead the Republican Party into the fall presidential campaign against Democratic front-runner Hillary Clinton — a stunning political triumph for a first-time candidate whose appeal to frustrated voters was widely underestimated.

Trump's victory in Indiana Tuesday and Ted Cruz's abrupt decision to drop out resolved the Republican nominee for 2016, but it still left the party in a deep state of uncertainty. Some Republican leaders remain acutely wary of the bombastic billionaire and have insisted they could never support him, even in a faceoff against Clinton.

Nebraska Sen. Ben Sasse, who has consistently said he could not support Trump, wrote on Twitter Tuesday that he was being asked if the Indiana results changed his views. "The answer is simple: No," Sasse wrote.

Republicans such as Sasse worry both about Trump's views on immigration and foreign policy and his over-the-top persona. Hours before clinching victory in Indiana, Trump was floating an unsubstantiated claim that Cruz's father appeared in a 1963 photograph with John F. Kennedy's assassin, Lee Harvey Oswald — citing a report first published by the National Enquirer.

Trump still needs about 200 delegates to formally secure the nomination, but Cruz's decision to end his campaign removed his last major obstacle.

"Ted Cruz — I don't know if he likes me or he doesn't like me — but he is one hell of a competitor," Trump said of his last fierce competitor, whom he had dubbed "lyin' Ted." Trump, in a victory speech that was much lower-key than usual, promised victory in November, vowing anew to put "America first."

Vermont Sen. Bernie Sanders eked out a victory over Clinton in Indiana, but the outcome will not slow the former secretary of state's march to the Democratic nomination. Heading into Tuesday's voting, Clinton had 92 percent of the delegates she needs.

"I know that the Clinton campaign thinks this campaign is over. They're wrong," Sanders said defiantly in an interview Tuesday night. But Clinton already has turned her attention to the general election.

She and Trump now plunge into a six-month battle for the presidency, with the future of America's immigration laws, health care system and military posture around the world at stake. While Clinton heads into the general election with significant advantages with minority voters and women, Democrats have vowed to not underestimate Trump as his Republican rivals did for too long.

Republican National Committee Chairman Reince Priebus declared the GOP race over, saying on Twitter that Trump would be the party's presumptive nominee.

"We all need to unite and focus on defeating @HillaryClinton," he wrote.

Indeed, Trump's first challenge will be uniting a Republican Party that has been roiled by his candidacy. While some GOP leaders have warmed to the real estate mogul, others see him as a threat to their party's very existence.

Even before the Indiana results were finalized, some conservative leaders were planning a Wednesday meeting to assess the viability of launching a third party candidacy to compete with him in the fall.

Indiana was viewed as the last gasp for Cruz, the fiery Texas conservative. He campaigned aggressively in the state, securing the support of Indiana's governor and announcing businesswoman Carly Fiorina as his running mate but lost momentum in the closing days.

Cruz had clung to the hope that he could keep Trump from reaching the 1,237 delegates needed for the nomination and push the race to a rare contested convention. But aides said he made the decision to drop out early Tuesday evening, shortly after most polls in Indiana had closed.

"I've said I would continue on as long as there was a viable path to victory; tonight I'm sorry to say it appears that path has been foreclosed," Cruz told a somber crowd in Indianapolis.

Ohio Gov. John Kasich is now the only other Republican left in the race. But Kasich has won just one primary — his home state — and trails Trump by nearly 900 delegates.

Kasich pledged to stay in the race, with his campaign manager saying the governor would continue to "offer the voters a clear choice for our country."

Only about half of Indiana's Republican primary voters said they were excited or optimistic about any of their remaining candidates becoming president, according to exit polling conducted by Edison Research for The Associated Press and television networks. Still, most said they probably would support the GOP winner.

Clinton, too, needs to win over Sanders' enthusiastic supporters. The Vermont senator has cultivated a deeply loyal following, particularly among young people, whom Democrats count on in the general election.

Sanders has conceded his strategy hinges on persuading superdelegates to back him over the former secretary of state. Superdelegates are Democratic Party insiders who can support the candidate of their choice, regardless of how their states vote. And they favor Clinton by a nearly 18-1 margin.

With Sanders' narrow victory Tuesday, he picked up at least 43 of Indiana's 83 delegates. Clinton now has 2,202 delegates to Sanders' 1,400. That includes pledged delegates from primaries and caucuses, as well as superdelegates.

Trump now has at least 1,047 delegates. Cruz exits the race with 565, while Kasich has 152.

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Pace reported from Washington. Associated Press writer Stephen Ohlemacher contributed to this report from Washington.