On Monday evening, Sen. Chris Murphy, a Connecticut Democrat who sits on the Foreign Relations Committee, sent out an alarming series of tweets.

“It's time to take Trump seriously as he keeps hinting, over and over, that he wants to go to war with North Korea,” Murphy tweeted. He went on to argue that the media’s tendency to dismiss Trump’s comments as bluster has been wrong before, and Trump’s persistent series of provocations, as well as his dismissal of Secretary of State Rex Tillerson’s efforts to find a diplomatic solution, suggest his views on North Korea are more fixed than critics realized.

In recent years, Murphy has emerged as a leading foreign policy voice among Senate Democrats, as well as something of a social media celebrity. His tweets rack up thousands of likes and retweets, in part because he uses the platform to make actual arguments, not simply distribute press releases. But his string of missives on Monday read different from his usual fare. They read scared, even desperate. “Many of us have begun to hear whispers of more serious war talk in and near the White House,” he wrote. Trump’s "‘calm before the storm’ comment sent chills.”

I spoke with Murphy on Tuesday about the threat of war with North Korea, what he was hearing from his contacts in the Defense Department, and what Congress could do to prevent Trump from impulsively attacking North Korea. A transcript of our conversation, lightly edited for length and clarity, follows.

Ezra Klein

I think, for many, Trump’s semi-daily North Korea tweets have faded into background noise. So explain to me why you’re becoming more, not less, alarmed.

Chris Murphy

The president has sent us very clear signals about his enthusiasm for military conflict with North Korea. But because our brains understand that to be cataclysmic, we assume there’s another agenda behind his rhetoric, or that he will be convinced otherwise. I think given the stakes of a potential strike against North Korea, we have to act under the assumption that he’s serious, and he’s given us a lot of reason to think he is.

Ezra Klein

The rebuttal to this I hear most often is that Trump is surrounded by advisers who clearly don’t want war with North Korea, and it’s their calmer statements, and not Trump’s morning missives, that offer the truest guide to administration policy.

Chris Murphy

We know many of Trump’s advisers counseled him to stay in the Paris [climate] agreement. He didn’t. Very few people thought he would go as far as he did on the Muslim ban. The president tends to do what he says, much to the chagrin of his antagonists.

Ezra Klein

That’s a depressing sentence to hear spoken aloud.

Chris Murphy

We want to delude ourselves into thinking the president is still a reality show contestant when he tweets and that the government runs separate from his Twitter feed. That’s not how it works. And remember, the chain of command in the military runs to the president of the United States, not to the secretary of defense. When Trump continues to talk in meetings about military action, whether or not Mattis agrees with him, it creates a momentum inside the chain of command.

Ezra Klein

One dimension of this that worries me is that a lot of people seem to be relying on a sort of soft coup to keep Trump in check. The hope, as I’ve heard it expressed, is that the generals surrounding Trump will control his paper flow, control the information he gets, and if Trump goes off the reservation and does try to order an unwarranted first strike, they will do something to stop it or slow-walk it or otherwise intercede. All of this might be better than Trump unleashed, but it seems to me to subvert civilian control of the military in a worrisome way.

Chris Murphy

I’m not sure I entirely buy the severity of that premise, but let me reframe it: If this ended up in the military convincing the president out of military action, it would be a dramatic reversal of traditional roles. From the Cuban Missile Crisis to Vietnam to the Iraq War, it is usually the military trying to convince the president to view a problem through a kinetic lens. The military sees problems through their toolkit. They are not very good at seeing complicated diplomatic paths out of vexing problems. To assume military leaders will tell Donald Trump not to use the military to solve a problem belies the history.

Ezra Klein

Your most worrisome comment, to me, was that you have “begun to hear whispers of more serious war talk in and near the White House.” Can you characterize those whispers any further?

Chris Murphy

I’m not going to go any deeper than that. We all have lanes into the Department of Defense, and in the last week or so, I and others that I trust have started to pick up a different tone from them and their representatives in Congress than we saw a few months ago. I’m not saying anyone is coming to Congress and presenting war plans. But we’re adept at picking up changes in tone, and there is one.

Ezra Klein

When you say you’re picking up a change in tone, are you saying you’re sensing more concrete planning for military action, or more fear that there will be military action?

Chris Murphy

I’m not willing to go any further than what I’ve said.

Ezra Klein

Let’s talk about the possibility of congressional containment on Trump. My sense is that both Republicans and Democrats in Congress broadly agree with you, and with much of the White House, that Trump should not be allowed to impulsively escalate us into a first strike with North Korea. Bob Corker, the chair of the Senate Foreign Relations Committee, said just this week that he was far from alone among Republicans who think Trump is dangerous. And yet there’s been very little concrete action by Congress to contain him or limit the damage he could do. Why?

Chris Murphy

Some of it is due to the simple atrophy of the muscles Congress traditionally uses to exercise authority on foreign policy. We haven’t passed a State Department authorization in a decade. We’ve not debated a war authorization in a decade and a half. When you don’t use your authority in meaningful ways for a long time, you forget how to do it.

Ezra Klein

The resistance to putting too much constraint on the president’s powers may be a historically useful norm, but is it really viable with this president?

Chris Murphy

It’s tough to preauthorize or de-authorize military authority. We don’t know the circumstances that would lead to escalation on the Korean Peninsula, and so it’s hard to write down the set of constraints around events we don’t know. But I think it’s important for Congress to authorize action against ISIS and the Syrian regime as a way of making the president see clearly that action against North Korea shouldn’t be taken without authorization.

Ezra Klein

So I understand what you’re saying, you’re arguing that Congress should authorize the military actions against ISIS and the Syrian government as a way of making clear to Trump that action against North Korea isn’t authorized?

Chris Murphy

I worry our failure to authorize action in the Middle East is a signal to the president that we have given him a blank check on overall military strategy. So I think it’s important to authorize action on ISIS or the Syrian regime because it may chill his interest in acting in an unauthorized way in the Korean Peninsula.

Ezra Klein

To be honest, that seems like an awfully subtle way to communicate with this president.

Chris Murphy

Agreed. But the alternative, which is to pass legislation pre-constraining his ability to manage foreign policy in the Korean Peninsula, is difficult as well.

Ezra Klein

When I think about this, I imagine historians writing about this era in the aftermath of a nuclear exchange with North Korea that killed millions. And if that happens, I think they’re going to look at us and ask, “What were you doing? He was tweeting ‘Rocket Man’ insults at one of the world’s least rational leaders! How could you not have stopped this?” And I don’t know what we’re going to be able to say.

Chris Murphy

That’s why I sent out that series of tweets. I worry we have normalized his behavior and in retrospect this will be part of a catastrophic series of events leading to crisis. But if Republicans and Democrats made publicly clear he did not have the authority to engage in an unauthorized strike on North Korea, I don’t think that would be insignificant.