Sen. John McCain (R-AZ) stated on Monday that he promises Senate Republicans "will be united against any Supreme Court nominee that Hillary Clinton, if she were president, would put up."

I will set aside the fact that McCain was a member of the 109th Congress's "Gang of 14" senators who stood together publicly to limit Senate obstruction of judicial nominations — ultimately securing an up-or-down (successful) confirmation vote for current Supreme Court Justice Samuel Alito — because the Democrats' use of the "nuclear option" in 2013 arguably rendered the agreement moot.

No, let's focus on the present: What is McCain seeking to achieve with this line-in-the-sand statement? It's clearly unreasonable to promise that the Senate Republicans would stand united to oppose any nominee that Hillary Clinton might offer. Suppose she nominated McCain. Is he saying he would decline the nomination? Perhaps he would accept the nomination and vote against himself?

Of course, that is a silly argument — but only at first. The Court currently has only eight members. So McCain is saying that if Clinton wins in November, the Court will not get a ninth justice for at least four years, regardless of whom Hillary nominates. (Of course, the Senate might confirm Judge Merrick Garland in a lame-duck session, but Utah's Republican Sen. Mike Lee doesn't think so.)

McCain, not a Donald Trump supporter, is — like House Speaker Paul Ryan — trying to insulate his party (and himself) against the downside possibilities of Trump's current troubles. He is saying, in effect: The only bulwark conservative voters have remaining against a Clinton administration is the Senate.

Of course, it would be interesting to know if McCain would be saying such things if he weren't up for reelection this year. It would also be interesting to know what other Republican incumbents up for reelection this year, such as Mark Kirk (R-IL) and Pat Toomey (R-PA) — for whom McCain was campaigning on Monday when he made the statement — would say if asked whether they would obstruct "any Supreme Court nominee that Hillary Clinton, if she were president, would put up."

This is interesting not just in partisan terms: Trump's campaign appears to have possibly fractured the GOP base. Specifically, Trump's campaign has seemingly tapped into a broad swath of voters who are suspicious of powerful insiders and conspiracies. They may hate Hillary Clinton and/or her Supreme Court nominees, but it's not entirely clear that they hate them so much more than the GOP establishment — one that has broadly, though of course not completely, disavowed Trump in recent weeks. In other words, McCain's position might amount to choosing the fire over the frying pan.

In any event, they are definitively unprincipled; one can state that a healthy GOP presence in the Senate will temper Clinton's influence on the Court, but it is irresponsible for a senior and accomplished politician — a former presidential nominee — to publicly endorse petulant opposition based only on "who the president is."