The just-married elder Baldwin, in the headlines in recent weeks for run-ins with photographers stationed outside his Greenwich Village home, said last year that he was ruling out a run for mayor in 2013. He said in December that he wasn't "horny" for the office like other candidates.
That announcement came after a raft of bad publicity following his meltdown on an American Airlines flight.
But Billy Baldwin said on CNN's "Piers Morgan Live" on Tuesday that his brother remains interested in the office.
"I think on one level it’s very real. I think he’s very committed and extremely bright and knowledgeable and I think he could do it and he could run,” Billy Baldwin said. “On the other hand, I’m not quite sure if he’s cut from the proper cloth because it requires a lot of tolerance and a lot of patience and…"
Morgan cut in to note that the elder Baldwin has a reputation for being "a bit hot-headed."
Billy Baldwin replied: "Let me remind you that John McCain and Rudy Giuliani are quite the same and they are very successful and highly effective politicians."
Baldwin returns to NBC's "30 Rock" in the fall for the show's abbreviated final season.
