Donald Trump holds a 15-point lead in the Republican presidential primary in Indiana, and a majority of GOP voters disapprove of the effort by underdogs Sen. Ted Cruz and Gov. John Kasich to coordinate a strategy to block him, a new Wall Street Journal/NBC News/Marist Poll finds.

Hillary Clinton holds a narrow, 4-point lead over Sen. Bernie Sanders among likely Democratic voters in Indiana, the poll found.

Ahead of the voting in Indiana on Tuesday, Mr. Trump is the first choice of 49% of likely GOP primary voters in the state, with 34% favouring Mr. Cruz and 13% for Mr. Kasich.

That gives Mr. Trump a substantial advantage in a state that Mr. Cruz says could be his last, best chance to stop the front-runner from clinching the GOP nomination before the Republican National Convention in Cleveland in July.

Mr. Trump has been on a winning streak since his landslide victories in the New York primary two weeks ago and in last week’s five-state round of voting in the Northeast, which some dubbed the “Acela primary.”

Among Democratic primary voters in Indiana, the poll found a close contest between Mrs. Clinton, who is the first choice with 50% of voters, and Mr. Sanders, with 46%.

While that outcome would bring a division of delegates between the candidates, it wouldn’t change the dynamic of a nomination fight that the former secretary of state is now almost certain to win.

“After the Acela primary, there is an aura of inevitability surrounding the Trump and Clinton candidacies,” said Lee M. Miringoff, director of the Marist College Institute for Public Opinion.

In the Republican primary, it is still mathematically possible for Mr. Cruz, of Texas, and Mr. Kasich, of Ohio, to pick up enough delegates between now and the end of the primary season on June 7 to keep Mr. Trump from reaching the 1,237 needed to clinch the nomination. But the path to doing so narrows substantially if Mr. Trump sweeps Indiana and wins all of its 57 delegates.

In a last-ditch effort to derail the Trump train, the Cruz and Kasich campaigns last week announced that they would try to coordinate a strategy to block Mr. Trump, a plan that called for Mr. Kasich to stop campaigning in Indiana to give Mr. Cruz a clearer shot at the front-runner.

The poll found that 58% of likely Republican primary voters disapproved of that Cruz-Kasich deal and saw it as “further proof that the Republicans are trying to rig the game against Trump.” One-third of GOP voters approved of the deal and “of doing everything possible to stop Trump from being the Republican nominee.”

It isn’t clear how much the Cruz-Kasich agreement will affect the outcome next Tuesday, as 63% of GOP primary voters said it wasn’t a factor in deciding their votes, including 53% of Kasich voters and 66% of Cruz voters.

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The poll also found that not all of Mr. Kasich’s supporters would accrue to Mr. Cruz if Mr. Kasich weren’t in the race.

Democratic front-runner Hillary Clinton spoke at a campaign event in Hammond, Ind., on April 26. The Journal/NBC News/Marist Poll found the former secretary of state holds a four-point lead over Sen. Bernie Sanders in the Indiana primary.

Democratic front-runner Hillary Clinton spoke at a campaign event in Hammond, Ind., on April 26. The Journal/NBC News/Marist Poll found the former secretary of state holds a four-point lead over Sen. Bernie Sanders in the Indiana primary. Photo: Jim Young/Reuters

If the Ohio governor’s support is reallocated according to the second choice of his backers, Mr. Trump still maintains an 11-point lead, with 53% backing him and 42% supporting Mr. Cruz.

Nearly two-thirds of Republican voters said that if no candidate wins a majority of delegates during primary season and the party winds up with a contested convention, then the candidate with the most votes in the primaries should get the nomination.

Opinions on that question varied according to which candidate a voter favoured for the nomination. Some 64% of Kasich supporters and nearly half of Cruz supporters said that if no one gets a majority, convention delegates should select the person they think would be the best nominee.

Looking ahead to the general election in Indiana, a state that has voted for a Democratic presidential candidate only twice since 1940, the poll found that Mrs. Clinton would lose to all three of the GOP candidates. She would lose 48% to 41% to Mr. Trump, 50% to 43% to Mr. Cruz and 56% to 37% to Mr. Kasich.

But Mr. Sanders would beat Mr. Cruz, 48% to 45%, and nearly tie the other two potential GOP nominees.

Mr. Trump’s provocative style rubs more Democrats the wrong way than Republicans or independents. Overall, 42% of voters consider his campaign statements to be frequently insulting and think that he has the wrong approach on many issues. That includes 73% of Democrats but only 38% of independents and 20% of Republicans.

Indiana Republicans on Tuesday will also decide a primary race for Senate to replace retiring GOP Sen. Dan Coats.