If Mr. Obama nominated a moderate and the Republicans who control the Senate refused to confirm the nominee, would the country find itself in the throes of a constitutional crisis? I did a quick poll of five constitutional law professors on Saturday night, and the consensus was no. “The world won’t crumble,” as Bruce Ackerman, a Yale law professor, put it.

The government will still function in the interim, even if it is a long interim. The court can still decide cases. If it splits 4 to 4 (as it does every once in a while when one justice or another sits out a case), then the lower court ruling is upheld. That is likely to happen in one major case this term. The court recently heard arguments in a significant challenge to the labor movement over the collection of dues by public-sector unions, and it looked as if the plaintiffs would prevail; a tie, without Justice Scalia, would keep the current system intact. Even a run of 4-to-4 rulings would not create chaos. The outcomes would be more likely to preserve the status quo, especially because the court could wait to take cases until it can resolve them with a clear majority.

But even if a Republican refusal to confirm a nominee by Mr. Obama would not bring the government to a stop, it would still be a major political struggle — a “stress test for our system of separation of powers,” said Richard Hasen, a University of California, Irvine, law professor and author of the new book “Plutocrats United.” In 2004, Mark Tushnet, a Georgetown University law professor, wrote an article about “constitutional hardball,” which he defines as legal and political moves that are “within the bounds of existing constitutional doctrine and practice but that are nonetheless in some tension with existing pre-constitutional understandings.”

In other words, as Matthew Yglesias of Vox put it, “moves that do not violate the letter of the law, but do trample on our conventional understanding of how it is supposed to work.” The rise of the filibuster, especially to obstruct the routine working of government, is a particularly good example: It is not unconstitutional or otherwise illegal, but it has become a sharpened partisan weapon. It also strains the traditional system of separation of powers, transforming the Senate from a cooling saucer into a freezer.

I asked Professor Tushnet if Mr. McConnell’s apparent plan to block an Obama nominee qualifies as constitutional hardball. He said “probably yes,” explaining in an email: “The argument for calling it hardball is that there might be a reasonably settled understanding that it’s undesirable for the Supreme Court to operate ‘too long’ without a full complement of judges, and, as a result of that understanding, that the Senate will consider nominations sent to it reasonably far in advance of a presidential election.”