A Libyan rebel fighter with a wounded leg is carried in the operating room at the Bir Muammar Hospital on the outskirts of Zawiyah. (BOB STRONG/REUTERS)

Defense Secretary Leon Panetta, when asked Tuesday whether the Libyan leader was any closer to being driven, said that his “sense is that Gaddafi’s days are numbered.”

It was a prediction that had a certain ring to it, and for good reason.

In July, Secretary of State Hillary Rodham Clinton observed of Gaddafi: “His days are numbered.”

In April, Mike Mullen, chairman of the Joint Chiefs of Staff, said: “I actually do believe his days are numbered.”

And in March, at the White House, Jay Carney concluded, “It’s becoming increasingly clear the Gaddafi’s days are numbered,” a remark the press secretary has made repeatedly since then.

It’s true that the “Mad Dog of the Middle East” has been defying predictions of his demise for months. (Or, for that matter, for decades.)

The question now is whether “days numbered” could become more than just a talking point among American officials.

It’s possible that’s the case.

The rebels have made critical gains, capturing the strategic town of Zawiyah and threatening to place a stranglehold on Tripoli. Gaddafi’s deputy security chief reportedly became the latest official to defect. And the Libyan government’s use of a Scud missile over the weekend might just be a sign of desperation.

But for experts, the question remains less when Gaddafi will leave but how.

The Libyan leader has used terrorism as an instrument of state policy, and he and his lieutenants have said from the get-go that they will fight until the end.

Among Libyan expats, at least, the belief is that Gaddafi isn’t just saber-rattling, said Stephen Flanagan, a fellow at the Center for Strategic and International Studies. “That is what everyone is dreading – this idea that you have this kind of Armageddon-like last-stand that he takes.”

The other key question, of course, is what Gaddafi would leave behind if he were to be ousted. It’s the same question that arose when mass demonstrations first erupted in Libya, and yet there are still no clear answers.

The United States and other Western government may have recognized the Transitional National Council as the legitimate government, but the opposition is still riven by divisions among various factions and tribes. There’s not much of a civil society or a political establishment without Gaddafi.

“At the end of the day, getting rid of Gaddafi will be the easy part,” said Christopher Boucek, an associate in the Middle East program at the Carnegie Endowment for International Peace.

Boucek said, from a policy point of view, it makes sense to keep peeling off key members of Gaddafi’s inner circle by convincing them that the costs of staying loyal to him are too high. But it also would help to start developing some real plans for the West to extricate itself from Libya.

Even if U.S. predictions are right — and Gaddafi’s days are numbered — there’s no guarantee that he’s going to run out of them in the immediate future. The NATO resolution authorizing force will run out at the end of September, and among some members of the alliance, there are signs of growing impatience.

Sept. 23 will mark six months since the start of the mission.