Leaders of a pro-Hillary Clinton political group threatened to fire employees who questioned its founder's criticism of Vermont Sen. Bernie Sanders, according to a former staffer.

Employees of American Bridge 21st Century, an opposition research-focused super PAC that backed Clinton's candidacy, "had their employment threatened when they complained" about the treatment of Sanders, said Will Urquhart, formerly an American Bridge video tracker.

Urquhart revealed the group's treatment of Sanders supporters in a pair of tweets on Tuesday. He was responding to statements by American Bridge founder David Brock, who publicly apologized on Tuesday for his treatment of Sanders.

"I recognize that there were a few moments when my drive to put Hillary in the White House led me to take too stiff a jab," Brock wrote in a post on Medium. "I own up to that, I regret it, and I apologize to you and your supporters for it."

Brock's network of political groups, including blog Blue Nation Review and American Bridge spinoff Correct the Record, attacked Sanders as a radical, single-issue candidate myopically focused on the supposed misdeeds of Wall Street.

Correct the Record even linked Sanders to unpopular British Labour Party leader Jeremy Corbyn and late Venezuelan strongman Hugo Chavez.

In a Feb. 2016 interview with Time magazine, Brock personally rattled off a host of other attacks on Sanders, alleging he was unqualified to be president, ignorant of U.S. foreign policy, and hypocritical on campaign finance issues.

Urquhart blasted Brock's apology for those attacks on Tuesday. "Instead of threatening employment, maybe staffers that complained [about Brock's] attacks on Bernie should have been listened to," he wrote.

Reached for comment, he declined to elaborate, citing the terms of his separation agreement. According to Federal Election Commission records, Urquhart was on American Bridge's payroll from Jan. 2015 until Feb. 2016.

A former American Bridge staffer who supported Sanders during the primaries told the Free Beacon that multiple employees felt targeted for their candidate preference.

"Among Bernie supporters at Bridge, there was a tangible fear of being targeted because of that support. It wasn't just me, I had others telling me they were biting their tongue out of fear," said the former employee, who requested anonymity due to fear of legal reprisal.

An American Bridge spokesperson did not immediately respond to a request for comment.