Donald Trump has warned Republicans there will be riots if the party tries to block him becoming its presidential nomination.
With at least three more states in his win column, Mr Trump is now the only candidate with a path to clinching the Republican nomination before the party's convention in July.
But he must still do better in upcoming contests to get the necessary 1,237 delegates, leaving some opponents with faint hope he can still be stopped.
In the US primary system, voters in each state determine how many delegates each candidate receives and the delegates then vote for the nominees at the conventions where each party selects its candidate.
"I still think it's a very realistic chance that nobody's going to have a majority of the delegates," said Henry Barbour, a senior Republican National Committee member who worked on Marco Rubio's delegate strategy until the Florida senator exited the race on Tuesday.
Mr Barbour said Mr Trump "doesn't deserve to be president", but also said he could ultimately support the billionaire if he "can convince me that he's presidential material".
Mr Trump cautioned that his supporters would revolt if he falls just short in the delegate count and loses in a rules fight.
"If you just disenfranchise these people, I think you would have problems like you've never seen before," he said.
Despite the deep concerns about Mr Trump within the Republican Party, there was little tangible action that indicated a way to stop the businessman's march towards the general election.
There was no rush among party leaders or donors to coalesce around senator Ted Cruz, the only candidate in the race with a long-shot chance of overtaking Mr Trump in the delegate count. A small group of conservatives moved forward with plans to meet on Thursday to discuss the prospect of rallying behind a third-party option, but no candidate had been identified to lead that effort.
Meanwhile, Democratic front-runner Hillary Clinton set her sights on a November showdown with Mr Trump. Her sweep of Tuesday's five primary contests was a harsh blow to rival Bernie Sanders, giving Mrs Clinton what her campaign manager described as an "insurmountable lead" in the delegate count.
"We are confident that for the first time in our nation's history, the Democratic Party will nominate a woman as their presidential nominee," Robby Mook wrote in a memo to supporters.
Mrs Clinton has at least 1,599 delegates to Sanders's 844. It takes 2,383 to win the Democratic nomination.
Mr Trump urged Republicans to view the party's nominating contest with the same sense of clarity. During a round of calls to morning television shows, he said some of the same Republican senators who publicly criticise him have called him privately to say they want to "become involved" in his campaign eventually. He also picked up an endorsement on Wednesday from Florida Governor Rick Scott.
Mr Trump also effectively killed the next Republican debate scheduled for Monday in Utah, saying "we've had enough debates". After Ohio Governor John Kasich said he would not debate without Mr Trump on stage, host Fox News scrapped the event.
Mr Trump has won 47% of the Republican delegates awarded so far, according to the Associated Press delegate count. He needs to win 54% of the remaining delegates to clinch the nomination by the time the primary season ends on June 7.