Sen. Elizabeth Warren told WGBH News that her colleagues in Washington are concerned about President Trump’s mental health, following a series of recent tweets sent by Trump targeting North Korean leader Kim Jong Un.

“There’s a lot of talk about it in D.C., and a lot of talk about the fact that he says one thing today and says something different tomorrow and says something very different the day after that — much of this is dangerous,” Warren said. “I mean, trying to taunt Kim Jong Un, that could go very badly for us and puts us in greater danger, and prevents from developing coherent strategies that we need, internationally and domestically.”

Earlier this week, Trump boasted on Twitter that his so-called "nuclear button" was larger than the one Kim claimed to have on his desk in a New Year’s speech.

Trump’s barrage of tweets have concerned lawmakers, including over a dozen Democrats and one Republican senator who met with a Yale university psychiatry professor to discuss the president’s fitness for office.

In January of last year, Warren and Senator Ed Markey (D-Mass) introduced the Restricting First Use of Nuclear Weapons Act to the Senate, which would “prohibit the conduct of a first-use nuclear strike absent a declaration of war by Congress.”

According to Warren, Republicans (who have a majority in both the House and Senate) need to step up to provide safety measures in case nuclear weapons become more than just rhetoric.

“Where are the Republicans? Where’s the Republican party?” Warren said. “There’s not only no Republicans standing up saying, 'Yeah, let’s do this. Let’s move it to the floor and have a vote on it,' there’s nobody even talking about it. There’s just silence.”

When asked by Boston Public Radio host Jim Braude if former Massachusetts Governor Mitt Romney could be that Republican if he became a Senator from Utah, Warren criticized Romney’s record.

“I don’t know,” Warren said. “I’ve met so many, I’ve seen so many Mitt Romneys over the past few years, I literally don’t know which Mitt Romney it would be who would be the Senator from Utah.”

Romney has previously signaled he would run if Senator Orrin Hatch retired. Hatch announced this week he will not seek re-election after serving more than 40 years.

To hear the full interview, click on the audio player above.