WASHINGTON (Reuters) - U.S. Republican presidential candidate Ben Carson threatened to leave his party and launch an independent White House bid on Friday, accusing Republican Party leaders of trying to manipulate who wins the 2016 nomination.

Carson’s threat came a day after party operatives said the race is so unpredictable that it could result in a contested convention in July when delegates meet to formally pick their presidential nominee.

Carson’s concern is that party officials will rally around a candidate of their choice and exclude him.

“The party should not be doing anything that is deceptive and under the covers,” Carson told reporters in Burlington, Iowa.

The retired neurosurgeon had led polls of Republican voters, but support has slumped in recent weeks as Carson has struggled to offer details of his foreign policy vision.

A poll of Republican voters in the early voting state of New Hampshire conducted by public television station WBUR found that Carson’s support has fallen to only 6.0 percent from 17 percent in mid-September.

A so-called “brokered convention,” in which no single candidate has a sufficient number of nominating delegates to become the presidential nominee in the November election, used to be a common feature of American politics, but there has not been one in more than 60 years.

Top party operatives, at a dinner hosted by Republican National Committee Chairman Reince Preibus, discussed the possibility that the Republican battle for a nominee will extend to the July 18-21 convention in Cleveland, officials told Reuters on Thursday.

They stressed that the issue came up only briefly.

In a statement, Carson expressed concern that party leaders would attempt to influence the nominating process.

“If the powerful try to manipulate it, the Republican National Convention in Cleveland next summer may be the last convention,” he said.

If there were plans for a brokered convention, he said, “I assure you Donald Trump won’t be the only one leaving the party.”

“Dr. Carson, don’t worry,” Republican National Committee spokesman Sean Spicer said on CNN. Spicer said the discussions concerned the delegate selection process. “It’s nothing more than that,” he said.

Donald Trump’s rise to the top of the 14-candidate field has confounded establishment Republicans who have been waiting in vain for the New York billionaire’s insurgent campaign to collapse.

Trump has repeatedly threatened to embark on an independent run for the White House if the party does not treat him fairly.