“This is something that highly motivates the Democrats’ base and something that motivates the Republican base, so it is pretty much we are back at the line of scrimmage,” said Senator John Cornyn of Texas, the No. 2 Senate Republican and a former chairman of the Senate Republican Campaign Committee.

Polls show that independents side with Democrats on the call to at least consider a nomination, but Mr. McConnell and his advisers are confident that those Americans will cast their ballots based on a range of issues rather than just the court.

The court fight also could prompt conservatives, who have not been that excited about Senate Republicans, to put real energy and money into the campaigns. To many in the anti-abortion and pro-gun movements, there is no bigger issue than maintaining conservative control of the Supreme Court. That potential makes the risk worth it to Mr. McConnell, even if it means discarding the considerable effort he made at reversing a Republican reputation for Senate obstructionism.

While Senate Republicans have essentially united behind Mr. McConnell’s approach, there are murmurings of discontent. Some say Mr. McConnell should have at least held a conference call among Republicans before assuming such a politically charged stance. But Mr. McConnell, worried about members — who were scattered during a recess — taking positions they might later have to reverse, felt he had to act decisively.

Perhaps the biggest question is why Mr. McConnell is drawing such a hard line against even meeting the nominee or conducting a hearing, and in doing so handing Democrats and editorial pages around the country the argument that Senate Republicans are refusing to do their fundamental duty. Republicans have become well practiced at slow-walking nominees and stalling legislation. Some are asking, why not just conduct hearings, review the nominee and then turn thumbs down?

Mr. McConnell and his team worry that once such a Senate process begins, it becomes unpredictable and hard to control, particularly if the nominee seems well qualified and reasonable. Their thinking is that it is better to stop the confirmation cold. When Mr. Obama announces his choice — probably within the next week or so — Republicans will argue that their resistance has nothing to do with that individual.

Mr. McConnell is widely recognized for his keen political instincts, and he may prove to have handled the court fight adroitly.