Hillary Clinton Hillary Diane Rodham ClintonHillicon Valley: FBI chief says Russia is trying to interfere in election to undermine Biden | Treasury Dept. sanctions Iranian government-backed hackers The Hill's Campaign Report: Arizona shifts towards Biden | Biden prepares for drive-in town hall | New Biden ad targets Latino voters FBI chief says Russia is trying to interfere in election to undermine Biden MORE's former campaign manager said Monday that President Trump's campaign aides should be "prosecuted for treason" if an FBI investigation proves they coordinated with Russia in the presidential campaign.

Robby Mook said he's "glad" FBI Director James Comey is testifying before the House Intelligence Committee, where Comey on Monday morning disclosed that his agency is investigating possible collusion between Trump's campaign and Moscow during the election.

"We need to find out exactly what happened," Mook said in an email to The Hill. "If US campaign aides helped the Russians, knew about the Russians plans, or asked the Russians to intervene in any way, they should be prosecuted for treason and measures should be put in place to make sure this never happens again."

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He said Russian President Vladimir Putin's goal was clearly trying someone "who he regarded as an enemy of his regime."

"He was punishing her for actions she took on behalf of the American people as our Secretary of State," Mook said.

Mook warned that if Russian interference is proved true, it sets a dangerous precedent for other nations to meddle in U.S. affairs.

"If we allow this practice to continue, Russia and other foreign nations could have a dangerously powerful influence on our legislative and executive branches," he said. "We complain all the time about the influence of super pacs and campaign donations in our politics, which are real, because they punish officials who act contrary to their interests. Now, foreign powers are attempting to gain a similar influence and they must be stopped."

Tracy Sefl, a Clinton surrogate, called the revelations from the hearing "sickening."

"There's a scene in Hamilton when Jefferson exclaims of his rival [Alexander] Hamilton, 'Well he's never gon' be president now. That's one less thing to worry about.' Well Trump is president and we now have this much more to worry about," Sefl said.

Another Clinton campaign aide said Comey's statement wasn't shocking to many inside the operation.

"It's what we've been saying all along, even during the campaign multiple times," the aide said. "The only problem is no one was listening at the time."

--This report was updated at 12:20 p.m.