WASHINGTON — Senator Mitch McConnell’s strategy to maintain the Republican majority has been clear: trying to prove that his party can govern. But by saying he will block a Supreme Court nominee who has not even been named, Mr. McConnell is headed toward partisan warfare instead.

The death of Justice Antonin Scalia has energized a right flank that has been long suspicious of Mr. McConnell and forced him into a fight that is likely to derail his smooth-functioning Senate. The tactic could alienate moderate voters and imperil incumbent Republicans in swing states, but in the supercharged partisanship of a Supreme Court fight, he probably had no choice. By framing his decision as deferring to voters in the next election, people close to him say he has minimized the political risk.

“It was necessary,” said Josh Holmes, Mr. McConnell’s former chief of staff, who now works as a Republican consultant. “The suggestion that the American people should have a say here isn’t exactly risky ground to be treading.

“As for the politics,” he continued, “if anyone thinks the center of the electorate is clamoring for Obama to name another left-wing jurist they’re nuts. The liberal left will be as loud as they ever have been, but the reality is that the consternation will be confined to the activist left.”