The night Justice Antonin Scalia died in February, Russell Wheeler with the Brookings Institution said there is nothing in the Constitution that says the Senate has to confirm, or even consider, the president's nominee.

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(The Constitution says the Senate "shall advise" the president, which over time we've taken to mean will approve or disapprove of. But despite precedent, Wheeler says there are very few courts that would interpret "shall" as "must.")

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And while there is a law that says there will be nine justices, it's been followed loosely. The court has sat with six, seven, even 10 justices at different points in history. Pew Research found that in the 19th century, court vacancies sat unfilled for months, or even years.

Holding up a Supreme Court nomination might be contrary to the way we've done things for decades, but there is nothing in our nation's laws that would stop Senate Republicans from doing it. President Obama's nominee Merrick Garland holds the record (by more than 100 days) of the longest wait for his nomination: 226 days and counting. As incensed as Democrats may be about that, they can't really file a lawsuit to try to get Republicans to consider Garland.

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Republicans can keep up the blockade when there is a new president (and likely a new Supreme Court nominee) too, Wheeler says. They could do it if they held onto the majority, and they could do it if Democrats take control of the Senate with a thin majority, one shy of the 60 votes it would take to prevent Republicans from filibustering a Clinton nominee. Clinton could, in theory, appoint a Supreme Court nominee while the Senate is on a break, but if Senate Republicans were in the majority, they could prevent a break from ever happening (at least, on paper).

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And don't count on Republicans nominating Garland after the election in a lame-duck session. Even Senate Minority Leader Harry Reid (D-Nev.) thinks that's unlikely.

If Clinton wins, Republicans have a political incentive to fight her nominee. Replacing one of the court's most conservative justices in recent history with a moderate or liberal one isn't the sort of Senate accomplishment a GOP incumbent would look forward to explaining to base voters back home.

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But just as could envision a world where the Supreme Court goes years with eight justices, we could envision a world where Republicans get punished at the ballot box for it.

For one, they would need to come up with a new rationale for blocking a new justice, since their current argument — the need to wait for the election of a new president — would be moot. Cruz seems to be testing out an idea others have floated earlier this year: It's not the end of the world to have eight justices.

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Republicans also have apathy on their side right now. Right now, the average voter does not seem to care much about the GOP blockade — at least, not enough to boot Republicans out of office for it. But that could change if days stretch into years without a ninth Supreme Court justice.

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Americans' apathy is in spite of the fact the current court has already opted not to take up some pretty big decisions on such issues as President Obama's immigration reform and teachers unions. Some of those non-decisions favored Republicans, others Democrats. But could you imagine another few years of deadlocked or non-decisions? That is objectively unsatisfying -- and it's possible that Americans might start to care about how many justices are on the Supreme Court.

Finally, if were other vacancies opened during President Clinton's presidency — a real possibility, since two justices are 80 or older right now — Republicans would have a harder time justifying a blockade of multiple nominees.

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There is one surefire way Democrats could short-circuit a Republican Supreme Court blockade. Go nuclear — specifically, change the rules (again) regarding how the Senate approves judicial nominations. In 2013, Senate Democrats changed the filibuster rules so that just a simple majority is needed to confirm most judicial nominations. But they left Supreme Court justices to the original 60-vote threshold.

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The outgoing Harry Reid has floated the possibility of lowering the bar to confirm Supreme Court justices. But he will have retired by January 2017, and his likely replacement, Sen. Chuck Schumer (D-N.Y.), doesn't seem to want to go that far yet.

"I hope we won't get to that," Schumer told CNBC's John Harwood. "And I'll leave it at that."

To sum up, McCain and Cruz are right: Senate Republicans can block a Supreme Court nominee for as long as they want. The question now becomes whether they are up for it.