On Sunday night, the U.S. Senate gained the fourth and final vote needed to block Donald Trump’s national emergency declaration when Rand Paul announced that he would support a Democratic measure to overturn the president’s executive action. The bill, which has already passed in the House, would then get sent to Trump’s desk—where the president would almost certainly issue a veto.

Paul, the “libertarian-ish” senator from Kentucky, offered a candid explanation for his decision to rebuke the president’s attempt to circumvent Congress to build a wall on the U.S.-Mexico border. “In September of 2014, I had these words to say: ‘The president acts like he’s a king. He ignores the Constitution. He arrogantly says, ‘If Congress will not act, then I must,’” Paul wrote in an op-ed for Fox News, referring to his opposition to Barack Obama’s executive order on immigration. “I would literally lose my political soul if I decided to treat President Trump different than President Obama.”

The defection will be keenly felt inside the White House. Paul has been described as the “Trump whisperer,” a fellow political iconoclast who enjoys a deeply personal connection to the president, despite their ideological differences. Though Paul has frequently spoken against Trump’s foreign policy agenda, such as air strikes in Syria, he has also been influential, encouraging the president’s anti-interventionist instincts. At other times, he has been a loyal Republican foot soldier, such as when he decided to support the nomination of Supreme Court Justice Brett Kavanaugh despite Kavanaugh’s past ruling in a mass surveillance court case.

Trump’s choice to circumvent Congress to fund his border wall, however, was apparently a bridge too far in Paul’s book:

I must vote how my principles dictate. My oath is to the Constitution, not to any man or political party. I stand with the president often, and I do so with a loud voice. Today, I think he’s wrong, not on policy, but in seeking to expand the powers of the presidency beyond their constitutional limits. I understand his frustration. Dealing with Congress can be pretty difficult sometimes. But Congress appropriates money, and his only constitutional recourse, if he does not like the amount they appropriate, is to veto the bill.

Paul joins Republican Senators Lisa Murkowski, Thom Tillis, and Susan Collins in supporting the resolution, which in all likelihood won’t actually put a stop to the state of emergency, but which represents an embarrassing rebuke of the president. Trump has already suffered numerous defeats in his quest for a border wall, many thanks to his own inability to negotiate. After turning down a $25 billion offer from Chuck Schumer early in his presidency, he shut down the government for more than a month to eke out a measly $5.7 billion for what he pitched as a barrier made of “steel slats.” Even if Trump vetoes the resolution, which is a virtual inevitability, Paul warned that the Supreme Court, filled with “the president’s own picks,” would very likely overturn his declaration.

Other Republicans, Paul said, should follow his lead, or risk becoming hypocrites of the highest order. “Every single Republican I know decried President Obama’s use of executive power to legislate,” he said. “We were right then. But the only way to be an honest officeholder is to stand up for the same principles no matter who is in power.”

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