Chuck Hagel’s fiercely resisted bid to become secretary of defense gained steam Thursday when Sen. Richard Shelby of Alabama, a senior conservative Republican, said he would vote for him.

"He's probably as good as we're going to get," Shelby told the Decatur Daily in his home state, according to the Associated Press.

In another boost, former Senate Republican leader and presidential candidate Bob Dole, a decorated World War II veteran, issued a statement Thursday saying, "Hagel's wisdom and courage make him uniquely qualified to be secretary of defense and lead the men and women of our armed forces. Chuck Hagel will be an exceptional leader at an important time."

Shelby's support was a clear sign of weakening Republican opposition, but Hagel's fiercest GOP foes struck back immediately. Fifteen Republican senators sent a letter was sent to the president calling on him to withdraw the nomination, asserting that Hagel lacks the bipartisan support needed to serve.

"The occupant of this critical office should be someone whose candidacy is neither controversial nor divisive," wrote the senators. Leading the effort was Sen. John Cornyn of Texas. Notably missing from the list was Sen. John McCain of Arizona, who has opposed Hagel but has said he wouldn't stand in the way of a Senate vote.

Hagel is expected to get all 55 Democratic votes and the support of three Republicans — Sens. Thad Cochran of Mississippi, Mike Johanns of Nebraska and Shelby. Two other Republicans — Sens. Susan Collins of Maine and Lisa Murkowski of Alaska — voted last week to allow the nomination to move ahead and are expected to do the same next week, giving Hagel the requisite 60 votes out of 100 necessary to end a filibuster.

An up-or-down vote on confirmation, with only a majority necessary, could come as soon as Wednesday.