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Donald J. Trump would be the weakest Republican presidential candidate to face off against Hillary Clinton in a general election matchup, according to a new poll that shows Senator Ted Cruz and Gov. John Kasich as stronger nominees against the Democratic front-runner.

According to a survey from Quinnipiac University, Mr. Trump continues to be the most popular candidate with Republican voters, but he trails Mrs. Clinton in a hypothetical matchup by a margin of six percentage points, 46 percent to 40 percent. The poll had a margin of error of plus or minus three percentage points.

Mr. Kasich, who has won only Ohio, his home state, so far, is the strongest against Mrs. Clinton, leading her by 8 points while Mr. Cruz trails Mrs. Clinton by 3 points. The poll finds that Senator Bernie Sanders, who is also facing long odds in his race with Mrs. Clinton, would be the strongest Democratic candidate in matchups against Mr. Trump and Mr. Cruz, and he is essentially tied with Mr. Kasich.

Mr. Trump widened his delegate lead over Mr. Cruz with a victory in Arizona on Tuesday night, but he continues to face an uncertain path to securing the delegates that he needs to win the nomination.

The prospect of a contested Republican convention, which could happen if none of the candidates have the necessary 1,237 delegates, has sparked a debate within the party over what should happen if Mr. Trump is close to that threshold but not quite there. He has suggested that there could be “riots” among his supporters if a candidate with fewer votes was handed the nomination by party leaders.

A separate poll from Monmouth University that was also released on Wednesday showed that 54 percent of Republican voters agree that Mr. Trump should be the nominee as long as he has the most delegates, while 34 percent say it should go to someone else. Mr. Cruz and Mr. Kasich are the most popular second and third options.

But the “Never Trump” movement continues to resist him. A majority of Republicans who are not supporting the Manhattan businessman are pushing for a contested convention in the event that he is leading but does not have enough delegates for a first-round ballot victory.

“That would probably throw the party into turmoil with many Trump supporters abandoning the party,” said Patrick Murray, director of the Monmouth University Polling Institute.