WASHINGTON — Donald Trump refused Sunday to rule out running as an independent if he fails to win the Republican presidential nomination, renewing a threat party leaders thought they had quashed.

“I want to run as a Republican. I will beat Hillary Clinton,” Trump said on “Fox News Sunday.”

When pressed to rule out an independent run, the New York billionaire said, “I’m gonna have to see how I was treated. It’s very simple.”

The prospect that Trump will launch an insurgent third-party run and draw his ardent supporters away from a rival Republican nominee, widening the divisions in the GOP, is a nightmare scenario for party leaders.

Some are supporting efforts to try to deny Trump the nomination if he cannot lock up enough delegates during the primaries to secure the nomination at the Republican National Convention, a movement Trump has complained is unfair.

The latest lurch in the rocky GOP race began Tuesday when Trump publicly abandoned the pledge he signed in September to support the eventual Republican nominee even if he loses. All the GOP candidates then in the race made similar vows.

At a town hall sponsored by CNN on Tuesday in Milwaukee, Trump was asked if he still felt bound by his pledge.

“No, I don’t anymore,” he said. Asked why, he responded, “I have been treated very unfairly.”

During the same town hall, Trump’s remaining rivals, Sen. Ted Cruz of Texas and Ohio Gov. John Kasich, also appeared to back off their pledges to support the GOP nominee. Neither was as direct as Trump, however.

Polls show Trump trailing Cruz in Wisconsin, which holds its primary Tuesday. But Trump appears on solid ground in New York and several other states that vote later this month, and he still could lock up enough delegates to win the nomination outright.

Reince Priebus, chairman of the Republican National Committee, warned Sunday that Trump’s backsliding on the pledge could discourage rank-and-file Republicans and party leaders from supporting him if he wins the nomination.

“Those kinds of comments, I think, have consequences,” Priebus said on ABC’s “This Week. “And so when you make those kinds of comments, and you want people to fall in line for you, it makes it more difficult.”

At stake for a potential nominee is a valuable database of contacts and party machinery to get out Republican voters in the general election.

“If you were running for president of the Kiwanis Club or the Boy Scouts and you said you don’t know if you like the Kiwanis or the Boy Scouts, I think that makes your challenge even greater to ultimately win those kinds of posts,” Priebus said.

“It’s not different for the Republican Party,” he added.

Priebus appeared on all five main TV talk shows Sunday, in part to tamp down allegations party leaders are trying to game the GOP’s arcane candidate selection rules to deny the nomination to Trump.

Priebus insisted the GOP would select its candidate fairly and openly, although he made clear Republicans are preparing for the possibility of a contested convention July 18 in Cleveland.

If no candidate secures 1,237 delegates on a first ballot, many delegates would be free to pick another candidate in subsequent rounds of voting. So Trump, who has won the most delegates so far but might not reach the required tally, still could lose the nomination.