Transcript for Trump's lawyer takes blame for problematic tweet
Good evening. And it's great to have you with us here on a Monday night. And we begin with president trump and his words for Michael Flynn, after his former initial security adviser pleaded guilty to lying to the FBI. Now cooperating with Robert Mueller. The president now saying, I feel badly for general Flynn, going on to say of the FBI and Flynn, that they destroyed his life. And tonight, the president and a new legal strategy from his team. Arguing a president cannot obstruct justice. ABC's chief white house correspondent Jonathan Karl leading us off. Reporter: At the white house today, the president offered words of sympathy for his former national security adviser, Michael Flynn. Well, I feel badly for general Flynn. I feel very badly. He's led a very strong life. Reporter: Over the weekend, a decidedly different tone from the president, when he was asked if he stands behind Flynn. Do you stand by Michael Flynn sir? We'll see what happens. Reporter: Flynn's plea bargain is a big red flag for the white house. He has agreed to cooperate fully with special counsel Robert Mueller. I think what we're beginning to see is the putting together of a case of obstruction of justice. Reporter: The president seems determined to undermine that case, by slamming the FBI, tweeting that its reputation is, quote, "In Tatters, worst in history." I will say this, Hillary Clinton lied many times to the FBI. Nothing happened to her. Flynn lied and they destroyed his life. I think it's a shame. T's very unfair. Reporter: Former FBI director James Comey has testified that he had no reason to believe Clinton lied to the FBI. He is defending the agency he once led, quoting his own words to congress, after he was fired by president trump. I want the American people to know this truth. The FBI is honest. The FBI is strong. And the FBI is, and always will be, independent. Reporter: Over the weekend, the president's own story seemed to change. He has said before that he fired general Flynn because he lied to the vice president. But this weekend he tweeted, quote, "I had to fire general Flynn because he lied to the vice president and the FBI." That tweet could become a problem. The president had not previously acknowledged knowing Flynn lied to the FBI when he fired him. James Comey says trump urged him to drop any investigation into Flynn, a possible obstruction of justice if the president knew at the time that Flynn had committed the crime of lying to the FBI. Now, the president's personal lawyer, John dowd, said it was he who wrote the president's tweet. But it has not been disavowed by the white house. And the president continues to deny that he ever asked Comey to go easy on Michael Flynn. And Jon Karl with us tonight from the white house. And Jon, the president's lawyer saying he wrote that tweet from the president's account. That got a lot of attention, tweeting for the president. Now, the lawyer says, I'm out of the tweeting business, I did not mean to break news. But in the meantime, Jon, the president's lawyers are going with a new legal strategy tonight when it comes to obstruction of justice? Reporter: The president's lawyer is arguing that the president, because of the nature of his job as the head of the executive branch, cannot, by definition, commit obstruction of justice. David, it's a variation of Nixon's line that if the president does it, it's not illegal. Jon Karl at the white house tonight. Jon, thank you.
This transcript has been automatically generated and may not be 100% accurate.