WASHINGTON, Dec 3 (Reuters) - A series of tweets by U.S. President Donald Trump about the investigation into contacts between his 2016 campaign and Russia prompted concerns on Sunday among both Democratic and Republican lawmakers, with Republican Senator Lindsey Graham saying Trump could be wading into "peril" by commenting on the probe.
"I would just say this with the president: There's an ongoing criminal investigation," Graham said on the CBS program "Face the Nation."
"You tweet and comment regarding ongoing criminal investigations at your own peril," he added.
On Sunday morning, Trump wrote on Twitter that he never asked former FBI Director James Comey to stop investigating Michael Flynn, the president's former national security adviser - a statement at odds with an account Comey himself has given.
That tweet followed one on Saturday in which Trump said he had fired Flynn because the former national security adviser had lied to the FBI.
Trump's attorney, John Dowd, said in an interview with Reuters that he had drafted the tweet and said putting it together had been a "mistake."
"Ill take responsibility," Dowd said.
The series of tweets came after a dramatic turn of events on Friday in which Flynn pleaded guilty to lying to the FBI and agreed to cooperate with prosecutors delving into contacts between Trump's inner circle and Russia before he took office.
Legal experts and some Democratic lawmakers said if Trump knew Flynn lied to the FBI and then pressured Comey not to investigate him, that could bolster a charge of obstruction of justice.
Senator Dianne Feinstein, the top Democrat on the Senate Judiciary Committee, said she believed the indictments in the investigation so far and Trump's "continual tweets" point toward an obstruction of justice case.
"I see it most importantly in what happened with the firing of Director Comey. And it is my belief that that is directly because he did not agree to lift the cloud of the Russia investigation. That's obstruction of justice," Feinstein said on NBC's "Meet the Press."
"The president knew he (Flynn) had lied to the FBI, which means that when he talked to the FBI director and asked him effectively drop this case, he knew that Flynn had committed a federal crime," Adam Schiff, ranking Democrat on the House Permanent Select Committee on Intelligence, told the ABC program "This Week."
The Russia matter has dogged Trumps first year in office, and this weekend overshadowed his first big legislative win when the Senate approved a tax cuts bill.
Flynn was the first member of Trumps administration to plead guilty to a crime uncovered by special counsel Robert Muellers wide-ranging investigation into Russian attempts to influence the 2016 U.S. election and potential collusion by Trump aides.
Russia has denied meddling in the election and Trump has said there was no collusion.
Comey, who had been investigating the Russia allegations, was fired by Trump in May. He told the U.S. Senate Intelligence Committee in June he believed his dismissal was related to the Russia probe, and said Trump asked him to end the investigation of Flynn.
"I never asked Comey to stop investigating Flynn. Just more Fake News covering another Comey lie!" Trump said on Twitter on Sunday.
On CBS, Graham criticized Comey, saying he believes the former FBI director made some "very, very wrong" decisions during his tenure as FBI director. But Graham also said Trump should be careful about his tweets.
"I'd be careful if I were you, Mr. President. I'd watch this," Graham said. (Reporting by David Morgan, Susan Cornwell in Washington and Karen Freifeld in New York; Writing by Roberta Rampton and Caren Bohan; Editing by Mary Milliken)