Rep. Maxine Waters Maxine Moore WatersPelosi: House will stay in session until agreement is reached on coronavirus relief Omar invokes father's death from coronavirus in reaction to Woodward book Business groups increasingly worried about death of filibuster MORE (D-Calif.), one of President Trump's most vocal critics, hit him on Twitter Friday after Michael Flynn’s guilty plea, urging him to “tell the truth” in the ongoing special counsel investigation into whether his campaign colluded with Russia.

Mr. President, there is help for your deteriorating mental health. It's called admitting guilt and telling the truth. — Maxine Waters (@RepMaxineWaters) December 1, 2017

Flynn, Trump's first national security adviser, pleaded guilty on Friday to lying to the FBI about his contacts with Moscow and reportedly will testify that Trump directed him to make contact with Russians.

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“Lock him up, lock him up, lock him up, lock him up,” Waters tweeted, mocking the chants that Flynn and Trump led on the campaign trail in reference to Democratic nominee Hillary Clinton Hillary Diane Rodham ClintonJoe Biden looks to expand election battleground into Trump country Biden leads Trump by 12 points among Catholic voters: poll The Hill's Campaign Report: Biden goes on offense MORE.

Lock him up, lock him up, lock him up, lock him up. — Maxine Waters (@RepMaxineWaters) December 1, 2017

Waters, who has repeatedly called for Trump's impeachment, also criticized him over former acting Attorney General Sally Yates’s testimony earlier this year that she warned the White House about Flynn.

Trump: Flynn admitted lying to the FBI. Sally Yates warned you that Flynn had been compromised by your friend Putin. You should've listened. Thank you Sally Yates! — Maxine Waters (@RepMaxineWaters) December 1, 2017

Waters has been a harsh critic of Trump, particularly on the topic of Russia, saying earlier this year that she “can guarantee [Trump] is in collusion with the Russians.”