Corker, the Republican chair of the Senate Foreign Relations Committee who announced in late-September that he wouldn’t be seeking reelection in 2018, never had a particularly strong relationship with Donald Trump. In May, he described the Trump administration as being in the midst of a “downward spiral.” Three months later he criticized Trump’s handling of Charlottesville, saying the president “has not demonstrated he understands the character of this nation.” But it wasn’t until Corker announced that he wasn’t seeking reelection, that the relationship between Trump and Corker was fully severed. On Sunday, after Corker had repeatedly criticized Trump’s handling of foreign policy, the president took to Twitter:

Bob Corker gave us the Iran Deal, & that's about it. We need HealthCare, we need Tax Cuts/Reform, we need people that can get the job done! — Donald J. Trump (@realDonaldTrump) October 8, 2017

Corker fired back:

It's a shame the White House has become an adult day care center. Someone obviously missed their shift this morning. — Senator Bob Corker (@SenBobCorker) October 8, 2017

Later that day, Corker told The New York Times’s Jonathan Martin and Mark Landler that the president’s “reality show” approach to governance was leading the country “into World War III. “He concerns me. He would have to concern anyone who cares about our nation.”

This is a significant development, given Corker’s influence as the chair of the Foreign Relations Committee. But it’s easy to overstate its significance. After all, we’ve been here before—Jeff Flake wrote an entire book criticizing Trump that just happened to be published the same week that Flake voted for a health care bill being pushed by the president that would have had disastrous consequences for millions of Americans. It does take some courage to criticize the president but, like Flake, Corker hasn’t yet done anything to meaningfully constrain Trump, either on foreign policy or anywhere else. Instead, his comments are best understood as an attempt to influence the president’s behavior—social media and press coverage, of course, are the two best ways to reach Trump and his inner circle.

In his interview with the Times, Corker made it clear that he thinks that most of his Republican colleagues in the Senate are just as concerned about Trump’s unstable behavior as he is. “Look, except for a few people, the vast majority of our caucus understands what we’re dealing with here,” Corker said. “If you write that, I’m sure there will be some that will come to you and say, ‘No, no, no. I don’t believe that.’ But of course they understand the volatility that we’re dealing with, and the tremendous amount of work that it takes by people around him to keep him in the middle of the road.”