Senator John McCain (R-Ariz.) en route to a Senate Republican policy luncheon on Capitol Hill March 28. (Melina Mara/The Washington Post)

The “nuclear” showdown in the Senate has split the chamber’s most senior Republicans over how bad the fallout will be.

On one side is Senate Majority Leader Mitch McConnell (R-Ky.), who says that once Democrats filibuster Judge Neil Gorsuch, the GOP’s unilateral response to confirm him on a simple majority vote will take senators “back to what was the tradition in the Senate” for confirming Supreme Court justices.

It will be, McConnell says, a good thing for the Senate.

“Look at the Senate through the long history of the body, the practical effect of all this will be to take us back to where we were,” McConnell, 75, told reporters Tuesday.

On the other side is Sen. John McCain (R-Ariz.), the fiery 80-year-old who sees this move as the next step in the inexorable slide to crushing the chamber’s bipartisan traditions. He thinks senators who view this as a good step are, well, not fully in command of their faculties.

(Peter Stevenson/The Washington Post)

“Idiot, whoever says that is a stupid idiot, who has not been here and seen what I’ve been through and how we were able to avoid that on several occasions,” McCain said Wednesday, recalling past efforts to defuse these judicial confirmation wars. “And they are stupid and they’ve deceived their voters because they are so stupid.”

Even so, McCain will support McConnell’s move to eliminate the 60-vote threshold for reaching a final vote on approving Supreme Court justices to lifetime appointments. That will come after Democrats, as expected, formally block Gorsuch’s nomination on Thursday.

There is no question that McConnell has the votes to change the Senate rules on a party-line vote. All 52 Republicans agree Gorsuch is a worthy nominee. They dismiss the Democratic opposition as a political reaction to McConnell’s refusal to consider Judge Merrick Garland’s nomination last year, and to the liberal base demanding resistance to anything President Trump proposes.

The dispute, instead, is about the future of the Senate. McCain and some senior Republicans assert that this will lead to the abolition of the filibuster in all its forms, including on legislation.

“This is a body blow to the institution, and I think we’re on a slippery slope,” McCain said.

McConnell promised to protect the supermajority on legislation, saying that filibusters on nominations are a new phenomenon but those on legislation are essential to the Senate’s essence.

Sen. Bob Corker (R-Tenn.), shown here at a rally for President Trump in Nashville, worries about blowing up Senate filibuster rules on legislation. (Jabin Botsford/The Washington Post)

“The legislative filibuster has been there a long time, everybody has been up or down, majority or minority at various times. We all understand that’s what makes the Senate the Senate,” he told reporters Tuesday.

It’s been 100 years since the Senate adopted Rule XXII, which was established because President Woodrow Wilson grew tired of a small band of senators blocking his efforts to execute World War I.

So the Senate formally established how a supermajority could shut off debate, setting a two-thirds threshold for ending a filibuster. After a bloc of southern Democrats obstructed civil rights legislation, a large bipartisan group in 1975 lowered that threshold to just 60 of the 100 senators — leaving in place the two-thirds majority for a rules change.

During George W. Bush’s presidency, Senate Democrats blocked more than 10 of his nominees to the appellate courts, the first time judicial nominees regularly got blocked by the minority. Sen. Charles E. Schumer (D-N.Y.), long before he became minority leader, drew up that strategy beginning in 2001.

This prompted Republicans to consider a unilateral move that would not technically be a rule change but would have the same effect. But McCain led a bipartisan “Gang of 14” to save the filibuster in 2005. At the time, then-Sen. Trent Lott (R-Miss.), who helped advise McCain’s group, dubbed it the “nuclear option” because it was such a partisan move that the fallout would be lasting.

Sen. Bob Corker (R-Tenn.), chairman of the Foreign Relations Committee, agrees with Lott’s prediction. In a little-noticed floor speech, Corker blasted unnamed senators who said the 2013 nuclear option by Democrats to end most filibusters on nominees and the pending Republican step were not bad for the Senate.

“Somebody acted like it was no big deal, that it had just gotten us back to where we had always been,” Corker said last week, appearing to direct his critique at McConnell.

He recounted how, after Democrats first triggered the nuclear option to end filibusters on most nominees, the Senate practically stopped working.

“There were days — not days, months — where people who had normally worked with people on the other side of the aisle just kind of shut down. It was hard to believe the nuclear option had been invoked,” Corker said.

Now, the sides have changed. McConnell has gone from hating the filibuster (2005) to supporting it (2013) to again trying to abolish it (2017) — an evolving set of positions that correlate with which party had the majority. Schumer has, coincidentally, held the exact opposite position of McConnell each step along the way.

“It’s not where you stand, it’s where you sit,” McCain said.

That’s why Corker argues that they will not be able to resist the temptation to blow up supermajority requirements on legislation.

“Everybody says, ‘Oh, we are never going to do it on legislation.’ Come on,” Corker said, adding that the pressure from the respective party bases will become too much. “Give me a break. Somebody is not living in reality.”

McConnell has tried to reassure his GOP colleagues that will not happen. He says that all presidential nominees will now be free of the filibuster hurdle, returning the chamber to its traditional role of voting them up or down.

“There’s not a single Senator in the majority who thinks we ought to change the legislative filibuster. Not one,” he said.

McConnell was then told about McCain’s sense that only a “stupid idiot” could say this was a good thing for the Senate. He laughed.

“I don’t think I have any comment on that,” McConnell said.

Read more from Paul Kane’s archive, follow him on Twitter or subscribe to his updates on Facebook.