(CNN)Sen. Richard Blumenthal said Monday that President Donald Trump's tweet over the weekend saying he fired former national security adviser Michael Flynn because he lied to the FBI is "very powerful evidence of obstruction of justice."
Blumethal: 'Credible case' for obstruction of justice against Trump
In a Saturday tweet, Trump said: "I had to fire General Flynn because he lied to the Vice President and the FBI. He has pled guilty to those lies. It is a shame because his actions during the transition were lawful. There was nothing to hide!"
The tweet raised questions as to whether Trump knew Flynn had lied to the FBI when he fired him in February. On Sunday, Trump's personal lawyer, John Dowd, said he was the person who drafted it.
On Monday, Blumenthal told CNN's Erin Burnett that the tweet is part of "a growing accumulation of evidence showing Donald Trump is potentially culpable of obstruction of justice."
"There is a credible case against him," he added.
Meanwhile, Trump's lawyer has defended the President by arguing that there are no legal grounds to convict the US President for obstruction of justice.
The "President cannot obstruct justice because he is the chief law enforcement officer," Dowd said during an interview with Axios.
But Blumenthal took issue with that defense.
"No president is above the law," he said. "For the President's counsel even to suggest that any President is above the law, really raises, again, a question of respect for the law, and the rule of law, and also potential contempt for the rule of law."