Sen. John McCain (R-Ariz.) did it again Thursday night. He asked supporters at a rally in Mesa, Arizona, to send him back to Washington so he can shore up a GOP majority that would “prevent that four-to-four split from tilting to the left.”

McCain was referring to the Supreme Court, and the possibility that a President Hillary Clinton could fill seats there ― starting with the one that has been vacant since Justice Antonin Scalia died in February. At the time, Republicans refused to consider President Barack Obama’s nominee, Judge Merrick Garland, by saying they should wait until the presidential election. This would “let the American people decide,” Senate Majority Leader Mitch McConnell (R-Ky.) said.

McCain’s statement is the clearest sign yet that Republicans are changing their minds. At this point, it’s pretty clear that a substantial and influential portion of the party is serious about blocking any Clinton nominees, thereby smashing a long-standing tradition of American government ― namely, that presidents should have leeway to appoint justices who share their political and philosophical perspectives.

This shouldn’t surprise anyone.

McCain first raised this possibility in the middle of October, during a radio interview on behalf of fellow Republican incumbent Sen. Pat Toomey of Pennsylvania. “I promise you that we will be united against any Supreme Court nominee that Hillary Clinton, if she were president, would put up,” McCain said at the time.

The comment instantly provoked backlash from Democrats ― and even a Republican or two ― who pointed out that a presumptive blockade of liberal Supreme Court nominations would represent a break with history. Although the Senate has constitutional authority to provide “advice and consent” on judicial appointments, it has rarely used that power to reject nominees for purely ideological reasons ― and, in modern times, only when the nominees held unusually extreme views.

If refusing to confirm nominees of different ideological beliefs became normal, there would be long stretches of open seats almost any time one party controlled the White House and the other controlled the Senate. This would potentially allow the court to shrink and create many more tie votes ― which, as The Huffington Post’s Cristian Farias has noted, could leave legal disputes between lower federal courts unresolved.

“We can’t just simply stonewall,” Senate Judiciary Chairman Chuck Grassley (R-Iowa) said following McCain’s remarks in October.

At first, McCain appeared to walk back his blockade pledge, when a spokesperson announced that the senator would “thoroughly examine the record of any Supreme Court nominee put before the Senate and vote for or against that individual based on their qualifications as he has done throughout his career.”

But that carefully worded statement didn’t actually disavow McCain’s promise to block any liberal nominees. And on Thursday night he returned to his original threat. Here’s the full quote:

I believe we must keep a majority in the United States Senate and one of the reasons is that there could be as many as three Supreme Court justices that there will be in the next four years. We have to have a Senate that will prevent that four to four split from tilting to the left and making decisions that will harm this nation for decades to come.

McCain isn’t the only one talking this way. Sen. Richard Burr (R-N.C.), who is facing a tough re-election fight, has vowed, “If Hillary Clinton becomes president, I am going to do everything I can do to make sure four years from now, we still got an opening on the Supreme Court.”

And then there is Sen. Ted Cruz (R-Texas), who has said he thinks the court can operate with fewer than nine justices if necessary.

I asked Cruz if there should be votes on Clinton court nominees if GOP holds Senate. He said there's plenty of precedent for <9 justices. — Dave Weigel (@daveweigel) October 26, 2016

Beyond Capitol Hill, allies of the Republicans are already outlining the intellectual justifications for adopting this posture. Rejecting nominees strictly for their political views fits well within the definition of “advice and consent,” Ilya Shaprio, senior fellow for constitutional studies at the libertarian Cato Institute, wrote recently in a column for the Federalist.

Shapiro is correct. But the question has never been whether the Senate can block nominees over ideology. It’s whether it should ― and under what circumstances.

The Senate has flexed that muscle only once in recent history. It was in 1987, when a Democratic Senate rejected President Ronald Reagan’s appointment of Robert Bork. Bork was an unusually polarizing figure ― somebody who, according to critics, was not just conservative but well outside the boundaries of mainstream judicial thought.

Ideological opposition to judicial nominations has risen steadily since that time, and it has come from both parties. A Democratic attempt to filibuster Samuel Alito’s nomination in 2006 famously included Obama, who was a senator back then. But that effort was mostly a gesture. Alito’s nomination was never really in doubt, any more than the nominations of Obama’s first two appointments, Sonia Sotomayor and Elena Kagan, were. All three sit on the bench today.

The ultimate disposition of Scalia’s former seat would appear to be a very different matter, with the likes of Burr, Cruz and McCain taking ideological opposition further than it has gone before. They are telling supporters that they will do whatever it takes to prevent the Supreme Court’s philosophical majority from shifting ― something that has happened throughout U.S. history, most recently when President George H.W. Bush appointed Clarence Thomas, a conservative, to replace Thurgood Marshall, a liberal, in the early 1990s.

Whether their argument will prevail is impossible to say. Last week Sen. Jeff Flake (R-Ariz.) joined Grassley in expressing skepticism of the McCain-Cruz position, telling Politico, “There’s a difference between what might be constitutional and what you could do politically and what you should do.” Sen. Tom Cotton (R-Ark.) has made similar, if somewhat ambiguous, comments.

But if Republicans have demonstrated anything in the last few years, it’s that most of them are willing to break political norms ― from allowing the federal government to breach its debt ceiling, to endorsing a nominee who believes in torture and brags about groping women ― if only to avoid punishment from their most conservative supporters.

Blockading Supreme Court nominees seems like an all-too-logical next step.