McCain says GOP will block any Clinton court nominee. Can they do that?

That vacant Supreme Court seat might stay empty for longer than expected if Democratic presidential nominee Hillary Clinton wins the election.

Speaking on a radio program in Philadelphia, Sen. John McCain (R) of Arizona, vowed that Republican senators would remain "united against" any nominee put forth by Mrs. Clinton, much as they’ve done to President Obama’s appointee Merrick Garland.

"I promise you that we will be united against any Supreme Court nominee that Hillary Clinton, if she were president, would put up," Senator McCain told 1210 WPHT radio Monday. "I promise you."

Republican senators have refused to hold a vote on any justice appointed by Mr. Obama, arguing that the next president should make the lifetime appointment rather than the current president finishing his last term. But McCain's latest words imply that Republicans plan to wage an ideological battle against even an incoming president's nominee, leading some to question how that strategy collides with the democratic process.

Rachel Dean, a McCain spokeswoman, told CNN that the senator will look at any nominee’s qualifications and vote based on what he or she has done prior to the nomination to make his decision.

"Senator McCain believes you can only judge people by their record and Hillary Clinton has a clear record of supporting liberal judicial nominees," she said. "That being said, Senator 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."

With one seat on the court open following the unexpected death of Justice Antonin Scalia and three other sitting justices in their 80s, it's likely that the next president will have the opportunity to make appointments that shape the court for decades to come. Despite Donald Trump's high ratings of unfavorability and serious doubts about his ability to serve as an effective president, some Republicans are encouraging their constituents to remain loyal to the GOP nominee, hoping that while he may be a risky choice for commander in chief, he’d appoint more conservative justices to the court.

McCain made the comments while speaking in support of Republican senator Pat Toomey, who is running in a tough re-election race for his seat in Pennsylvania.

"This is where we need the majority and Pat Toomey is probably as articulate and effective on the floor of the Senate as anyone I have encountered," McCain said. "This is the strongest argument I can make [for Mr. Toomey’s re-election] … so we can make sure there is not three places on the United States Supreme Court that will change this country for decades."

But McCain's push goes beyond partisan loyalty and could enter a realm of threatening the democratic process, critics say. By opposing the Supreme Court choice put forth by a democratically elected president, Republican senators could trigger a breakdown in the balance of power created by the current nominating system, Ian Millhiser, senior fellow at American Progress, writes for Think Progress.  

What reason would Democratic governors have to obey the decisions of such a [Republican nominated] court? What reason, for that matter, would a future Democratic president have to obey that court’s decisions? It’s one thing to ask the people’s elected leaders to bow to the decisions of unelected officials chosen in a fair process. It’s another thing altogether to ask them to bend to the will of a rigged bench.

McCain, in other words, is threatening a very dark future if Republicans keep their Senate majority. He is threatening to intentionally trigger a constitutional crisis where a bench stripped of its legitimate authority tempts defiance from elected officials who’ve been empowered by the electorate to govern.