Richard Gates has emerged as a key figure in special counsel Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE's investigation of Russia's interference in the presidential election.

Mueller on Thursday unveiled new criminal charges against Gates and Paul Manafort Paul John ManafortOur Constitution is under attack by Attorney General William Barr Bannon trial date set in alleged border wall scam Conspicuous by their absence from the Republican Convention MORE, his longtime business partner and President Trump Donald John TrumpUS reimposes UN sanctions on Iran amid increasing tensions Jeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Trump supporters chant 'Fill that seat' at North Carolina rally MORE's former campaign chairman, accusing them of a series of financial crimes including tax evasion, bank fraud and money laundering.

Observers for weeks had been expecting that Gates, a onetime Trump campaign aide, might strike an agreement with Mueller to avoid going to trial.

The superseding indictment released Thursday deepens the legal peril for Gates.

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Seth Waxman, a former federal prosecutor, said the timing of the filing likely aims to maximize the pressure on Gates, in case he is wavering on whether to cooperate with the investigation.

“It is designed to continue to ratchet up the pressure on Manafort and Gates to flip,” he said.

Waxman said the new filings suggest Mueller is likely sending a clear signal to Gates “that he is not messing around."

Gates is a key figure for Mueller because of his close relationship with Manafort.

Gates and Manafort worked hand-in-hand for more than a decade, including on lobbying work for a pro-Russia political party in Ukraine, so his testimony at trial could prove especially damaging for Manafort.

“If Manafort really did have a second in command comparable to a close confidant, you don’t get much better than that,” said Steven Cash, a lawyer at Day Pitney who specializes in criminal litigation and national security-related matters. “It’s like having the second in command of a company testify against the CEO.”

A plea agreement with Gates would put heavy pressure on Manafort to cut a deal of his own. If he flipped, it’s possible he could give Mueller inside information about any contact between the Trump campaign and Moscow during the summer of 2016, a key period when Russia began orchestrating the release of hacked Democratic emails.

“The Manafort indictment is serious, is ugly and is very likely backed by large volumes of evidence,” said Ron Hosko, a former FBI assistant director. “Gates can drive a final nail.”

Trump has flatly denied any collusion between his campaign and Russia, describing the allegations as a “hoax.”

Mueller’s team first unveiled an indictment against Manafort and Gates in late October.

That indictment, which included bank transactions, accused Manafort and Gates of money laundering, bank fraud and other financial crimes stemming from work overseas that pre-dated their involvement in Trump’s presidential campaign.

Both men pleaded not guilty last year.

Former prosecutors say Gates’s testimony, should he choose to cooperate, could help prosecutors sort through their evidence against Manafort. He could also fill in holes in timelines and help walk prosecutors through some of the complex financial transactions that he and Manafort are accused of carrying out.

“In most businesses and white-collar cases, the government really needs a cooperator to explain events,” said Jack Sharman, a former Whitewater special counsel.

The possibility of Gates on the witness stand could be enough to get Manafort to cooperate with Mueller’s prosecutors, which is likely their main goal.

Manafort worked for Trump’s campaign for six months before resigning under pressure in August 2016 over reports of his dealings with a pro-Russia oligarch in Ukraine. He was in the room for a June 2016 meeting at Trump Tower with a Russian lawyer who had promised Donald Trump Jr. Don John Trump'Tiger King' star Joe Exotic requests pardon from Trump: 'Be my hero please' Zaid Jilani discusses Trump's move to cancel racial sensitivity training at federal agencies Trump International Hotel in Vancouver closes permanently MORE damaging information on Democratic nominee Hillary Clinton Hillary Diane Rodham ClintonJeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Momentum growing among Republicans for Supreme Court vote before Election Day Warning signs flash for Lindsey Graham in South Carolina MORE’s campaign, and could provide inside information about any contact between the campaign and Russia.

If Gates were to strike a plea deal, he would be the fourth known witness to cooperate with Mueller’s investigation, in addition to former national security adviser Michael Flynn, former Trump campaign adviser George PapadopoulosGeorge Demetrios PapadopoulosTale of two FBI cases: Clinton got warned, Trump got investigated Trump says he would consider pardons for those implicated in Mueller investigation New FBI document confirms the Trump campaign was investigated without justification MORE and a California man ensnared in the probe for selling bank account numbers to foreigners committing identity fraud.

“I think the question is not are there going to be more indictments and guilty pleas. I think there will be,” Sharman said.

“The larger question is ultimately what is the relation of any of those offenses to the original core mission of the special counsel, who was appointed because of concerns over presidential campaign involvement.”