Senator Rand Paul opposed the nomination of outgoing Central Intelligence Agency director Mike Pompeo to serve as secretary of state for all the right reasons.

Then Rand Paul supported the Pompeo nomination for all the wrong reasons.

The Kentuckian’s rejection of his own arguments against putting a Koch-brothers errand boy in charge of American diplomacy confirmed that the senator is just another Republican who talks big about breaking with the Trump administration only to fall in line when it counts. Like Tennessee Senator Bob Corker and Arizona Senator Jeff Flake, Paul goes out of his way to present himself as his own man (and gladly accepts praise for his “independence”), but he refuses to deny Trump and the Republican leadership his vote on critical tests of conscience.

The tragedy in Paul’s betrayal of the system of checks and balances—as it is outlined in the Constitution he claims to regard so highly—is that the senator made the very best argument against voting for confirmation. As a member of Congress, Pompeo established himself as an unthinking militarist who showed scant regard for diplomacy or a constitutionally defined understanding of how wars can and should be initiated.

But it was Paul who formally confirmed that Pompeo was every bit as irredeemable as critics feared. When Paul, who holds a critical seat on the Senate Foreign Relations Committee, questioned Pompeo on April 12, the exchange exposed the fundamental threat posed by Trump’s nominee to replace ousted secretary of state Rex Tillerson. Paul raised concerns regarding Trump’s authority to order an air assault on Syria. “Do you think it’s constitutional? Does the president have the constitutional authority to bomb Assad’s forces?” he asked. “Does he have the authority absent congressional action to bomb Assad’s forces or installations?”

Pompeo answered: “Those decisions are weighted. Every place we can, we should work alongside Congress to get that but yes I believe the president has the domestic authority to do that. I don’t think—I don’t think that has been disputed by Republicans or Democrats throughout an extended period of time.”

“Actually,” replied Paul, “it was disputed mostly by our founding fathers, who believed they gave that authority to Congress, and actually they’re uniformly opposed to the executive branch having that power. In fact, Madison wrote very specifically, he said, ‘The executive branch is the branch most prone to war. Therefore, we have with studied care vested that authority into the legislature.’ So the fact that we have in the past done this doesn’t make it constitutional and I would say that I take objection to the idea that the president can go to war when he wants, where he wants.”

Paul’s point was well made. And, it should be noted, Pompeo was not merely wrong on the constitutional question. He was wrong on the facts. Members of Congress—from California Democratic Congresswoman Barbara Lee to Michigan Republican Congressman Justin Amash, from Senate Republicans like Utah’s Mike Lee to Senate Democrats like Virginia’s Tim Kaine—have for many years and in many circumstances raised concerns about undeclared and unauthorized military adventurism on the part of presidents of both parties.

Pompeo’s answers to questions from Paul and other members of the Senate Foreign Relations Committee were atrocious. And Paul’s agitating against Trump’s nominee was poised to tip the balance on the closely divided committee against endorsing Pompeo’s nomination. That would have been a rare rebuke by this Republican-controlled Senate for this Republican president—and an equally rare rebuke of executive overreach in the long history of presidential nominations of State Department leaders.

As The New York Times noted before Monday’s vote, “The Senate Foreign Relations Committee has never given a nominee for secretary of state anything but a favorable vote since the committee began considering nominees in the late 19th century, the Senate historian said…”

Then Rand Paul tossed off his constitutional concerns and voted for Pompeo, claiming that he was suddenly assured that the nominee had recognized that the Iraq War had been a bad move. Paul made repeated attempts to explain himself. “Having received assurances from President Trump and Director Pompeo that he agrees with the President on these important issues,” the senator announced, “I have decided to support his nomination to be our next Secretary of State.” He said that the president and his nominee had each agreed with the view that “we’ve been at war in too many places for too long.” In some interviews, Paul went further, describing issues where he felt he had found at least a measure of common ground with the administration. He even suggested that: “I have been assured by the president that there will be a discussion… of things the administration can do to have Fourth Amendment protections for Americans.”

But, of course, presidents and nominees are often quite agreeable when trying to swing a key committee vote. And this president has been notoriously inconsistent and unreliable when it comes to making assurances in such circumstances.

The fundamental fact remains that, after promising to do “whatever it takes” to block Pompeo, Paul removed a critical roadblock to Pompeo’s advancement.

In combination with equally shameful abandonments of principle by Democrats such as North Dakota Senator Heidi Heitkamp, Paul gave Trump’s nominee the boost that the president demanded.

Paul shamed himself. But he did something far worse than that, something that was far more damaging. The senator squandered a teaching moment that could finally have opened up the debate he claimed to want on the abuse of presidential powers. For those of us who have argued for decades that wars must be declared by Congress, not initiated at the whims of presidents, the prospect of that debate offered a rare glimmer of hope in a period of misdirection and danger. It seemed possible—never certain but at least possible—that a bipartisan coalition of conscience might finally assert itself as an alternative to the imperial presidency.

Paul abandoned that hope in order to curry the favor of Donald Trump. Paul betrayed himself, of course. But who cares about that? What really matters is that, at a critical moment for the Republic, Rand Paul betrayed the cause of the Constitution.