ROBERT SIEGEL, HOST:

There are big items on the White House agenda - changing the tax code, dealing with North Korea, trying to modify the nuclear deal with Iran. And not for the first time, it's President Trump's Twitter account that is stealing headlines. He has attacked ESPN, he's gone after NBC and he has traded digs with little Bob Corker, whom we at NPR generally refer to as Senator Bob Corker on first reference. And here to talk about all of this is NPR national political correspondent Mara Liasson. Hi, Mara.

MARA LIASSON, BYLINE: Hi, Robert.

SIEGEL: Let's start with the president's most recent target, NBC News. Trump accuses them of making up a story that said he wanted to increase the U.S. nuclear arsenal. And he tweeted this. (Reading) With all of the fake news coming out of NBC and the networks, at what point is it appropriate to challenge their license? Bad for country, exclamation point. How do you take that threat?

LIASSON: Well, the president has been on a slippery slope around the news media for quite some time. During the campaign, he said he would like to open up the libel laws. He's called the press the enemy of the people. Recently, he said the Senate Intelligence Committee should investigate the fake news media instead of the hoax Russia scandal. He said he should get equal time from late-night comedians. Now he's suggesting that maybe the broadcast licenses should be taken away from news outlets that he doesn't like.

So for someone who has taken copious advantage of his First Amendment rights as a reality TV celebrity, a political candidate and now as president, he just doesn't like it when the shoe is on the other foot.

SIEGEL: But assuming that this was more than mere bluster, we should add that it's not altogether clear that the president has the authority to follow through on this threat to NBC.

LIASSON: Well, certainly not because NBC as a network doesn't have a broadcast license to revoke. Their affiliates do, the stations do. I suppose he could try to take away their licenses. It would be very rare and unusual, and there's no indication yet that he has asked the FCC to do that. But just imagine for a minute if Barack Obama had said Fox's broadcast license should be revoked because what they said about him.

SIEGEL: Yeah. Well, let's turn to the back and forth between the president and Republican Senator Corker, a Republican of Tennessee. Trump blamed Corker over the weekend for the Iran deal. And he said that Corker had begged him for an endorsement. Corker's retiring. And Corker, for his part, tweeted, (reading) it's a shame the White House has become an adult daycare center. Someone obviously missed their shift this morning. I don't think we've ever talked about an exchange like this...

LIASSON: Na-na-na-na-na (ph). Yeah, right.

SIEGEL: ...Between a sitting president and a senator of his own party.

LIASSON: Yeah, it is really something. But it's important to remember that Bob Corker is not a Republican never-Trumper. He was one of Trump's few establishment Republican supporters. Corker was a businessman before he came to the Senate, and he saw Trump, I think, as a kindred spirit. And he gave him some real establishment endorsements in foreign policy. But Corker did recently say that Trump hadn't demonstrated the competence or stability to be a successful president.

Then after reports that Secretary of State Rex Tillerson had called the president a moron, Corker said that Tillerson was one of the people standing between our country and chaos. So after that, Donald Trump, as he does, counterpunched 10 times harder and he resorted to, as you mentioned, his favorite schoolyard insult, calling Corker little, like little Marco Rubio or little rocket man, Kim Jong-un. Not quite sure what he hoped to accomplish with that.

SIEGEL: What do you think, is Senator Corker an outlier, just, you know, one guy who's retiring and has decided to rebuke Trump? Or is he an omen of things to come in the Republican Party?

LIASSON: I think right now he's an outlier, at least in public. Corker, as you said, isn't running again. So he has the liberty to say things that other Republican senators say only in private. They don't want to get crosswise with Trump's base because those are the exact same voters who make up a disproportionate share of the Republican primary electorate. So outside of the Trump critics, like John McCain, Lindsey Graham, Ben Sasse and Jeff Flake, the rest of the Republicans in Congress are mostly keeping their concerns, which are similar to Bob Corker's, unspoken.

And I think that will continue until the 2018 elections or at some point when they may decide that the president is actually a drag on their political prospects.

SIEGEL: That's NPR national political correspondent Mara Liasson. Mara, thanks.

LIASSON: Thank you. Transcript provided by NPR, Copyright NPR.