Sen. John McCain (R-Ariz.). (Ross D. Franklin/Associated Press)

IT IS profoundly depressing to watch Americans of honor and goodwill surrender to toxic partisanship and become part of the problem. Listen, for Exhibit A, to Sen. John McCain (R-Ariz.) on Monday.

“I promise you that we will be united against any Supreme Court nominee that Hillary Clinton, if she were president, would put up,” Mr. McCain said on a Philadelphia radio show. “I promise you,” he repeated. Before the past year, the notion that presidents deserve a measure of deference in selecting Supreme Court nominees was among the nation’s essential political norms.

Then, Senate Republicans ignored President Obama’s nomination of Merrick Garland, among the best Supreme Court picks of the past half-century, supposedly on the principle that the next president should get to fill the Supreme Court’s open seat. Now Mr. McCain is suggesting that that justification was simply pretext for a partisan blockade that he would like to continue.

Rachael Dean, Mr. McCain’s communications director, attempted to soften the senator’s statement later Monday, writing in an email that Mr. McCain “will, of course, 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 Ms. Dean also wrote that “you can only judge people by their record and Hillary Clinton has a clear record of supporting liberal judicial nominees,” implying once again that no Clinton nominee would be acceptable. Ms. Dean did not respond when we asked what Mr. McCain would do if Ms. Clinton renominated Mr. Garland.

Even if Ms. Clinton tapped someone considered more liberal than Mr. Garland, that nominee would deserve a fair hearing. Senators should accept presidential nominees unless they are either truly unqualified or true ideological extremists. The functioning of government depends on speedy and open-minded judicial confirmations. In the past, Mr. McCain, who ran for president in 2008 and recognizes the importance of a sound appointment process, was a voice of restraint on these matters. Now he recklessly encourages Republican voters to expect that GOP senators will refuse any Democratic Supreme Court nominee.

This is a dangerous road. If the Republicans keep the Senate majority next month, acting on such an expectation will establish the precedent that the judicial branch can be staffed only when the president and the Senate are of the same party. If the Democrats take control, GOP intransigence could lead them to quash the filibuster on Supreme Court nominees, which would further politicize the judiciary and poison the process.

Like many Republicans, Mr. McCain hit a professional low when he endorsed Republican presidential nominee Donald Trump this year, an endorsement he retracted only after seeing a video of Mr. Trump boasting about sexually assaulting women. His vow to further politicize the judicial nomination process is another sad marker.