Former White House counsel John Dean is warning that former Trump campaign adviser Richard Gates's testimony could be the "end" of the Trump presidency.

"Mueller is throwing everything he can against Manafort, including Gates who can nail him. Increasingly it appears Manafort is the link to Russian collusion," Dean, who served in the Nixon administration, tweeted.

"If Gates can testify that Manafort was acting with Trump’s blessings, it’s the end of his presidency. That’s substantial."

Mueller is throwing everything he can against Manafort, including Gates who can nail him. Increasingly it appears Manafort is the link to Russian collusion. If Gates can testify that Manafort was acting with Trump’s blessings, it’s the end of his presidency. That’s substantial. https://t.co/t6pHNno1xL — John Dean (@JohnWDean) February 25, 2018

Gates pleaded guilty last week as part of a deal with 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 in his investigation into Russian election interference.

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He pleaded guilty on two federal charges brought against him by Mueller's team — one count of conspiracy against the United States and one count of making false statements to the FBI agents investigation Russian interference.

As part of the plea deal, Gates agreed to cooperate "fully, truthfully, completely, and forthrightly" with the special counsel's office as well as other law enforcement officials, according to court documents.

Speculation had mounted recently that Gates would plead guilty and cooperate in Mueller’s probe, making him a key witness who could testify in the criminal case against Manafort. The development is expected to ratchet up pressure on Manafort to cooperate in Mueller’s probe.

Dean said in a subsequent tweet that people have "expressed concern in this Manafort thread that Trump will pardon him."

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"Many of the counts in both the VA and DC indictments have state law counterparts that can be charged in NY and VA, where Trump had no pardon power. Checkmate is coming for 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," he tweeted.

A number of folks have expressed concern in this Manafort thread that Trump will pardon him. Many of the counts in both the VA and DC indictments have state law counterparts that can be charged in NY and VA, where Trump had no pardon power. Checkmate is coming for Paul Manafort. — John Dean (@JohnWDean) February 25, 2018

Dean has in the past warned he believes Trump is "more dangerous" than Nixon ever was.