His decision was part practical politics, part sentimentality. The two men were close in the Senate, having both served in Viet Nam, and saw eye to eye on other issues as well.
But ultimately, McCain counted noses and realized an attempt to filibuster the nomination would probably fail.
Sen. John McCain appears to have cleared the way Monday for Chuck Hagel to be the next secretary of defense.
The Arizona Republican, who has been a prominent voice in the debate over Hagel, said Monday he would oppose any attempt to filibuster the nomination, likely dooming any attempt by Senate conservatives to sustain a protracted procedural fight to delay Hagel's confirmation.
"I do not believe that we should filibuster," McCain told POLITICO. "To vote against is entirely the judgment of each individual senator, but a filibuster I think would be inappropriate."
Asked if he would vote for cloture if a filibuster were mounted, McCain answered, "Yes."
The White House and Democratic leadership are already confident that a solid majority exists for the Hagel to be confirmed on a simple majority vote. McCain's opposition to a filibuster should make it easier to get the 60 votes needed to invoke cloture.
Republican Sens. Mike Johanns of Nebraska and Thad Cochran of Mississippi have already announced they would support Hagel, meaning that Democrats appear to have at least 57 senators ready to support his nomination if they can keep their caucus united.
Indeed, there appears to be a real reservoir of concern in the GOP about filibustering against a Cabinet nomination. In interviews Monday, several senators expressed concern with such tactics, and McCain's statement could embolden more to speak out.
Sen. Pat Roberts (R-Kan.) has said he will oppose Hagel but doesn't want to raise the ante. "I don't want to filibuster. We don't want to go that way," he said. "It is a choice that could lead to a lot more problems."
Sens. Chuck Grassley (R-Iowa) and Susan Collins (R-Maine) also expressed reservations about any filibuster but were not ready to commit to a cloture vote.
A full court press by conservatives to derail the Hagel nomination might have succeeded if any Democrats, concerned about the nominee's positions on Israel and Iran would have broken ranks. But with with a solid phalanx of Democrats behind him, Hagel's nomination will go through - possibly with more than 60 votes.