Gov. Chris Christie of New Jersey ascended to become one of the country's top political stars almost in spite of his weight — dismissing those who questioned whether his size disqualified him for higher office as "ridiculous" and "irresponsible."

Now, like some overweight politicians before him, Mr. Christie has embarked on a major effort to shed pounds at a time when his eyes are on the presidency. Saying that "a whole bunch of other things" had not worked, he revealed on Tuesday that he had undergone weight loss surgery three months ago. He is now 40 pounds lighter.

Mr. Christie, a Republican, said concerns about his family, not politics, drove him to undergo the "Lap-Band" procedure, in which a silicone band is placed around the stomach to discourage overeating.

His staff and even his closest political advisers said they had not known he had the operation or had even considered having it.

But in the image-conscious world of politics, the questions about what effect slimming down would have on a possible presidential run immediately dominated the discussion of his move. Mr. Christie's heft has been part of his brand. Would he risk losing some of his Everyman appeal along with the weight?

The governor swatted those questions again. "Whatever size I happen to be when I have to make decisions about what to do next in my career, I doubt that will play any role or effect in what I decide to do," he told reporters massed outside an event in Newark. "It's not a career issue for me; it is a long-term health issue for me."