John McCain gave away the show.
"I promise you," Arizona's senior senator said Monday, "that we will be united against any Supreme Court nominee that Hillary Clinton, if she were president, would put up." A McCain aide later tried to walk back the pledge, saying 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 if you look carefully, you'll note that there wasn't really a contradiction: A Republican Senate might "examine" and hold hearings on Clinton nominees but that doesn't mean they would vote to confirm one.
What McCain said on Monday is almost certainly an honest account of what Republicans plan to do — that is, create a constitutional crisis should Hillary Clinton win the presidency and the GOP retain control of the Senate. The Supreme Court could be stuck with eight members for years, unable to resolve many crucial divisions in the federal courts. If the norm that presidents should be able to nominate qualified, mainstream judges who generally share their constitutional views disappears, the Constitution leaves no way to resolve the issue and staffing the federal government when the Senate and White House are in the hands of different parties will become increasingly difficult.
McCain's comments, first of all, should underscore that it's massively unlikely that Merrick Garland, Obama's nominee to fill the seat on the Court left vacant by the death of Antonin Scalia, will be confirmed during a lame-duck session. Republican senators will be under intense pressure not to collaborate with a Democratic president after what is likely to be a crushing defeat in the Electoral College. Throughout Mitch McConnell's tenure as leader of the Republican conference, Senate Republicans have consistently refused to make deals with Democrats even at the price of leaving substantial policy concessions on the table. Getting a slightly older and less liberal justice than might be confirmed otherwise is not the hill this practice is going to die on.
The more interesting question is what happens if Hillary Clinton wins the White House but Republicans maintain control of the Senate. This is possible — as of this writing, Nate Silver's FiveThirtyEight gives Republicans roughly a one in four chance of retaining the Senate, and two weeks ago it was closer to a 50-50 proposition. The conventional wisdom has been that it will be impossible for Republicans to keep Clinton from filling Scalia's seat for four years.
As McCain's unguarded comments indicate, this is dead wrong. Serial Republican obstruction of a Democratic replacement for Scalia is, in fact, entirely thinkable. The key question is this: What causal mechanism can force Republicans to confirm any Clinton nomination to the Court? They will surely get criticism from the press, but so what? Even if this particular form of obstructionism makes Senate Republicans marginally less popular, the electoral map in 2018 is so favorable to the GOP that it almost certainly wouldn't stop them from adding to their majority. The typical Republican senator has much more to fear from a primary electorate if a Democratic justice who would immediately become the swing justice creating majorities for liberal Supreme Court decisions was confirmed because of their vote.
One thing the conventional wisdom can't explain is why the extraordinary and unprecedented obstruction of Merrick Garland has been an utter non-issue in the presidential campaign. Regardless of whether the Supreme Court should be an important issue to most voters, in practice it isn't. Many Senate Republicans, having gotten away with it for a year, will assume they could get away with again — and they're probably right. It's true that congressional Republicans have eventually cut deals to end government shutdowns or to avoid defaulting on the national debt, but those are issues with direct, easily discernible material consequences to the public at large. The typical voter notices if they can't get into a national park or if there's massive economic collapse. They won't notice if the Supreme Court is failing to resolve circuit splits.
It's not certain that a Republican Senate would continue the Supreme Court blockade for another four years — we know the old norms are no longer operative but we can't be sure what new ones will be established. But it's entirely possible, and indeed likely.
What if Democrats retake the Senate? In this case, Republicans will almost certainly filibuster any Clinton nominee if they have the opportunity. The filibuster has already been eliminated for all other federal judicial and executive branch appointments, and Senate Democrats need to be prepared to eliminate it for Supreme Court appointments as well. If she wins, President Clinton should not re-nominate Merrick Garland, which would essentially reward Republicans for their obstructionism, but nominate someone younger and more liberal instead.
Republican Senate candidates in tight races might be distancing themselves from Trump, but collectively they're nonetheless holding a Supreme Court seat open for him. It looks like Trump will not get the chance to make any nominations — but Senate Republicans are very likely to do everything they can to ensure that Hillary Clinton can't fill the existing Supreme Court vacancy either.