BERLIN, Md. — Donald Trump’s supporters converged Wednesday on this small town on Maryland’s Eastern Shore for a massive rally, more confident than ever that he was closing in on the nomination and threatening retribution against Republican Party leaders if they stop him.

Many of the thousands of Trump voters at the rally, some of whom drove hours from Baltimore or Philadelphia, said they agreed with Mr. Trump that the Republican Party “rigged” the primaries against him and that the popular vote should rule the nomination process.

“Who gets the votes should win,” said Hannah Fizer, 20, a student at nearby Salisbury State University and a die-hard Trump supporter.

“They want to do everything in their power to stop him, but they can’t because he’s got that silent majority on his side,” she said, echoing Mr. Trump’s claim that he has turned the silent majority into a “noisy majority.”

With his blowout win a day earlier in his home state of New York, Mr. Trump has put so much distance between himself and his rivals that in an ordinary campaign season he would be the prohibitive favorite. Instead, he still faces bitter resistance from his own party leaders.

“This whole thing for delegates is rigged,” Mr. Trump told the boisterous crowd in the gymnasium at a high school in Berlin. He wondered aloud how it was possible that he could win the Louisiana primary and end up with fewer delegates than someone else.

“Here’s the good news: We’re ahead by like 300 delegates,” said Mr. Trump.

He also railed against the party establishment for bombarding him with negative ads, although he relished his victories anyway.

“Politics is a very dirty, dishonest business,” he said.

Mr. Trump is expected to sweep Tuesday primaries in Maryland and four other Northeastern states: Pennsylvania, Connecticut, Delaware and Rhode Island. The Republican race then heads to a battle royal May 3 in Indiana, which likely will be the “NeverTrump” coalition’s best chance to stop the real estate mogul.

Mr. Trump held a rally earlier in the day in Indianapolis.

Christina Warden, 38, a bartender who drove nearly three hours from Baltimore to the Maryland event, said Mr. Trump should get the nomination if, as expected, he keeps winning states in the final weeks of the campaign.

“If fair is fair, then he will get it — if [the Republican leaders] don’t pull something shady,” she said. “This is going to change the Republican Party as we know it.”

Ms. Warden and others said they would sit out the general election, vote third party or abandon the Republicans altogether if Mr. Trump is denied.

“If he doesn’t get the nomination, the party will implode,” she said.

Republican campaign strategist Ford O’Connell said Mr. Trump struck “political gold” when he rebounded from a demoralizing loss in the Wisconsin primary April 5 by adopting the narrative that the party system was rigged.

“It fired up his supporters, made Cruz look like a tool and redirected the conversation toward the system,” he said. “Every time we are talking about the system and the process, and Trump not winging it, then Trump is winning.”

Mr. Trump drew an enthusiastic audience in this conservative rural Eastern Shore community, which also boasts a large tourism trade from Ocean City and other seaside destinations. About 3,000 people gathered inside Berlin’s Stephen Decatur High School for the event and thousands more waited outside.

The event also drew protesters from far and wide, including members of the state teachers union, the liberal group Progressive Maryland, Black Lives Matter and others.

“It’s a laundry list of people who follow Trump,” said Berlin Police Chief Arnold R. Downing.

Mr. Trump’s burst of momentum prompted him to temporarily adopt a more presidential manner and eliminate some of his harsh rhetoric.

At his press conference after the New York win, he didn’t refer to Mr. Cruz as “Lyin’ Ted” as he usually did for the past couple of weeks but, in a dramatic departure, called him “Sen. Cruz.”

The new temperament didn’t last a day as Mr. Trump revived the “Lyin’ Ted” moniker in Indianapolis.

“Lyin’ Ted — lies. Oh, he lies,” he said. “You know Ted. He brings the Bible, holds it high, puts it down, lies.”

The name-calling was in response to Mr. Cruz’s claim that none of the candidates would collect the 1,237 delegates needed to clinch the nomination and that he and Mr. Trump are headed toward an open nominating convention.

“I’m millions of votes ahead — millions — millions of votes ahead of Lyin’ Ted Cruz,” Mr. Trump said. “I’m about 300 delegates ahead of Lyin’ Ted.”

After New York, Mr. Trump led the delegate hunt with 845 or the 1,237 delegates needed to win the nomination. The senator from Texas had 559 delegates, and Ohio Gov. John Kasich had 148.

Though he didn’t win any delegates in New York, Mr. Cruz previously proved masterful at working the system to pick off delegates from the front-runner. He is trying to force a contested convention in July in Cleveland, where he hopes to win the nomination after multiple ballots.

The election results in New York, where Mr. Cruz finished third behind Mr. Kasich, eliminated any possibility that the Texan can win the nomination with an outright majority on the first ballot, as every presidential nominee for decades has done.

But he can still keep Mr. Trump from securing the 1,237 delegates needed to clinch the nomination before Cleveland.

The Republican Party establishment has thrown its support behind Mr. Cruz as its last hope to block Mr. Trump’s unconventional run to the nomination.

• Seth McLaughlin and David Sherfinski in Washington contributed to this report.

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