It is no secret that Donald Trump casually disregards the United States Constitution whenever it gets in the way of his ego-trip presidency. But now, the president is threatening one of the few senators who truly respects the Constitution and who puts its dictates into practice.

After Montana Senator Jon Tester, the ranking Democrat on the Senate Committee on Veterans’ Affairs, and many other advocates for veterans began discussing concerns that had been raised last week regarding Trump’s nomination of White House physician Ronny Jackson to serve as secretary of veterans affairs, Jackson withdrew his name from consideration rather than face an embarrassing committee hearing.

Trump was furious—not with Jackson for buckling under pressure rather than answer questions about his background and vision, not with the two dozen current and former colleagues who raised concerns regarding Jackson, not with the media accounts that explored those concerns, and not with White House staffers who failed to prepare the president or Jackson for the rigorous confirmation process that Senate Veterans’ Affairs Committee members from both parties were duty bound to perform. No, Trump was furious with Tester for upholding a system of checks and balances that only works if members of the Senate are prepared to ask tough questions about presidential nominees.

A presidential Twitter storm attacked Tester as “very dishonest and sick!” and said “Tester should lose [his reelection] race in Montana.” After the Secret Service said it could not confirm a CNN report regarding Jackson’s reckless behavior, Trump declared: “Tester should resign.” The president veered so far out of control that he began spewing threats. “I know things about Tester that I could say, too. And if I said them, he’d never be elected again,” Trump announced to a Michigan crowd on Saturday.

That’s not presidential language. That’s the crude language you’d expect to hear from a deliberately intimidating and ill-informed villain in a classic gangster movie. Or from Joe McCarthy at the peak of his frenzies in the 1950s. Trump is attacking a man he knows nothing about. And it will not end well for the president.

Montanans who know Tester respect him as one of the Senate’s straightest shooters—a family farmer who got into politics to speak up for those who are often forgotten by politicians. Tester has always championed the cause of veterans. That’s what he was doing when he discussed the issues that had been raised with the Jackson nomination. Tester commented on allegations that had been forwarded to the committee, as well as on news reports about what a former White House Medical Unit staffer told CNN was “definitely inappropriate” behavior on Jackson’s part. Yet, Tester refused to pressure Jackson to withdraw. He encouraged the nominee to answer questions and appear before the committee, telling reporters, even as the White House sent conflicting signals regarding the Jackson nomination, that “there’s a possibility he could be confirmable.” What Tester pressed for was a hearing in which “we need to get to the facts.”

When asked about how Tester’s approach to the Jackson nomination, a spokesperson for Senate Veterans’ Affairs Committee chair Johnny Isakson, R-Ga., replied: “Senator Isakson has a great relationship with Senator Tester. He doesn’t have a problem with how things were handled.”

Former Republican senator Chuck Hagel, a Vietnam veteran who would go on to serve as secretary of defense, rejected the attacks on Tester, arguing there is “no one who cares more about veterans and looks out for their interests than my former colleague, Jon Tester.”

Tester has earned this bipartisan praise because, to a far greater extent than many of his colleagues, he has served as a senator is supposed to serve. When President Trump forwarded for Senate consideration the name of an unvetted and ill-prepared candidate to lead an essential agency with a $180 billion budget and almost 400,000 employees, the senator from Montana did not shy away from the duty the Constitution rests with him.

Motivated primarily by his concern for the veterans who rely on the VA for care—and whose continued care is threatened by the profiteers who seek to privatize major functions of the agency—Tester gathered information and discussed concerns that had arisen regarding a presidential nominee. He called for transparency and a serious review of that nomination. He refused to bend to political pressure, even though he faces what could be a tough reelection race in a state that handed Trump a 20-point advantage in 2016.

Tester used his position to assure that the interests of veterans would be protected. He did not leave things to chance. He did not simply trust a White House that treated the nomination of a secretary of veterans affairs as a casual afterthought.

That’s what the Constitution says senators are supposed to do. That Donald Trump would attack and threaten Tester, using the language of a thug rather than a president, tells us everything we need to know about Trump’s disregard for the separation of powers that sustains this country’s system of government. It also tells us why senators like Jon Tester are essential to the future of that system.