WASHINGTON – Donald J. Trump once again advanced toward clinching the Republican presidential nomination Tuesday, warning the GOP that if it tried to stop his candidacy, it could face “riots.”Trump significantly expanded his delegate lead, winning four out of five contests and narrowing the candidate field to three after beating Florida Sen. Marco Rubio in his home state.With decisive victories in Florida, Illinois and North Carolina on Tuesday, the bombastic businessman from New York has secured roughly half of the 1,237 delegates needed to automatically clinch the GOP nomination at the party’s July national convention in Cleveland, Ohio.A failure by Trump to reach 1,237 during the first ballot of a delegate vote would produce a contested convention, the only hope for the “Never Trump” movement.Trump said on Wednesday such a scenario would lead to “riots,” as he would enter the convention with the clear plurality of delegates and votes.“I don’t think you can say that we don’t get it automatically,” he told CNN. “I think you’d have riots. I think you’d have riots. I’m representing many, many millions of people.”An open convention could also allow for a nominee outside of the current candidates. Ohio’s governor, John Kasich, won his home state handily, in his first victory since the primaries and caucuses began in January.Kasich said his victory means he will remain in the race until the convention. Trump appeared to win the night’s fifth race in Missouri by less than 2,000 votes, over Texas Sen. Ted Cruz.Trump has locked down an estimated 646 delegates, compared to 397 for Cruz and 142 for Kasich.The winner in Missouri wins 12 additional delegates to the convention, above those proportionally allocated by congressional district.Every delegate will matter in this unique election, as GOP establishment figures remain committed to preventing Trump from securing the requisite majority.Political experts are divided over whether Trump is helped or hindered by a fragmented field. Kasich by remaining in the race, may deny Cruz critical votes in winner-takeall contests. Kasich’s 10 percent showing in Missouri, for instance, might have been enough to put Cruz over the top, if the Ohio politician hadn’t been on the ballot.Others argue that Kasich has kept critical delegates from going to Trump – including 66 in winner-take-all Ohio – and may continue to do so in future vote-proportional allocation states. And Kasich may yet become the candidate of choice for the “Never Trump” movement, including those in the establishment and moderate wings of the Republican Party, who hope to stop him from winning delegate-rich states such as Pennsylvania, New York and California.Victories in those states, as well as a consistent performance in other proportional- delegate states, would be enough to secure Trump the nomination. Trump announced on Wednesday that he will not be attending the next Republican debate, due to be hosted by Fox News on Monday, because of a scheduling conflict. He instead will address the American Israel Public Affairs Committee, an engagement “scheduled a while ago,” he said.“I’m making a very major speech in front of a very important group of people,” he told the network.Shortly afterward, Kasich, and then Cruz, said they wouldn’t debate if Trump didn’t appear. Fox then canceled the event.Every election year, AIPAC invites leaders from both parties to address the conference, one of the largest in Washington. US Vice President Joe Biden, House Speaker Paul Ryan and Democrat front-runner Hillary Clinton are also scheduled to address the confab.On the Democratic side, Clinton won victories in Florida, Illinois, Ohio, Missouri and North Carolina on Tuesday that cast doubt on US Sen. Bernie Sanders’s ability to overtake her for the party’s nomination.Her decisive victories gave her an almost insurmountable edge, but Sanders vowed to fight on.“With more than half the delegates yet to be chosen and a calendar that favors us in the weeks and months to come, we remain confident that our campaign is on a path to win the nomination,” he said.Reuters contributed to this report.