Sen. Rand Paul (R-Ky.) talks to reporters on April 7. (J. Scott Applewhite/AP)

After the final confirmation vote on Judge Neil M. Gorsuch, Sen. Rand Paul (R-Ky.) walked into a crowd of reporters to inveigh against the airstrikes in Syria. He’d been doing so nonstop since Thursday night — first in a statement, then on TV, then in a column decrying the “unconstitutional rush to war.”

In the Senate, his first question from a reporter was simple: Was President Trump abandoning his campaign’s foreign policy promises?

“It’s easier for me to give you my opinion than his,” Paul said before laying out a vision that sounded like campaign-era Trump. “When we toppled Saddam Hussein, well, he was a bad person who gassed the Kurds. We thought, we’ll get rid of him and things will get better. We toppled him, and it ended up empowering Iran. When we topple Assad, what comes after him?”

Moments later, Paul headed for an elevator and Sen. Lindsey O. Graham (R-S.C.) took his place. The senator, whose 2016 presidential bid felt at times like a way to argue with Paul and Trump, described an emotional late-night phone call with the president — a president who as a candidate had humiliated him by giving out his number. There were aspects of Trump “that remind me of Ronald Reagan,” Graham said.

“When Rand Paul talks about ‘this is illegal,’ he has no idea what he’s talking about,” Graham said. “There is absolutely no requirement, in my view, to go to Congress.”

Until this week, and despite Trump’s early personnel moves, libertarian skeptics of foreign intervention thought they had an ally in the White House. But in the space of 24 hours, Paul — who came off a golf trip with Trump with praise for the president — found the anti-interventionist Republican posse shrunken back to pre-Obama levels.

Paul, also a 2016 presidential candidate, was seen for several years to be the avatar of a “libertarian moment.” With President Barack Obama in the White House, Republicans and their voters were growing more skeptical of foreign intervention. The autumn of 2013 seemed to present a pivot point, when most Republicans rejected Obama’s call for airstrikes in Syria. Sen. Ted Cruz (R-Tex.), a rival for the libertarian vote, said that attacking Syrian airfields could turn the United States into “al-Qaeda’s air force” and wondered whether Obama was asking Congress to approve military action because “his bluff was called.”

The vast majority of Republicans who opposed Obama’s 2013 strike plan have come out in favor of Trump’s — which, analysts acknowledge, was more limited in scope. An analysis by the Denver7 news channel found that every Republican member of the congressional delegation who had been critical of the Obama attack, some even calling it “unconstitutional” without congressional approval, had backed Trump.

Cruz’s reaction was instructive. In a statement Thursday night, he said that “any military action in Syria must be justified as protecting the vital national security interests of America” but did not question Trump’s right to carry it out.

“I’m sure there will be ongoing discussions with the administration and Congress concerning last night’s attack, the legal justification, and any proposed military action going forward,” Cruz said Friday after leaving a briefing for senators. “I’ll wait to assess the administration’s argument.”

To Congress’s libertarians, Trump’s decision to act without consulting Congress was an obvious contradiction of how he had campaigned. On a Friday episode of former congressman Ron Paul’s online show, Rep. Thomas Massie (R-Ky.) hammered home the increasingly fringe — but once Republican mainstream — position that striking Syria without congressional authorization risked blowback.

“ISIS is better off today than it was yesterday,” Massie said, using another name for the Islamic State.

“That’s right,” Paul said. “That is the real shame.”

Rep. Justin Amash (R-Mich.), who joined Friday’s briefing for senators, said that Trump had been clear up until he became president. He had repeatedly spoken, tweeted and recorded short videos against the idea of bombing Syria.

“Back in 2013, the situation was fairly similar,” Amash said. “President Obama suggested that he might attack, and [Trump] opposed it.”