Sources close to the former president said that Mr. Clinton had been mulling such a defection for weeks, as early as the night of the Iowa primary, but that he only decided to make his decision public today.

“The American people want change,” Mr. Clinton said at a press conference in New York. “Lord knows I do.”

The former president said that “sometimes, at the end of a race, you have to put an old horse down,” adding, “I’m not speaking metaphorically.”

Mr. Clinton fueled speculation that he was seeking a role in an Obama administration, saying, “I know my way around the Oval Office, and I know how the super-secret double-lock works.”

The former president said he would relish a return to the White House, calling his tenure there “good times.”

For her part, Sen. Clinton said that the defection of her husband would not deter her from staying in the race, adding, “To my knowledge, he’s the only white voter Sen. Obama has.”

The New York senator denied that she was playing the race card, arguing, “Every other member of my family is supporting me, and by the way, they’re white.”

Elsewhere, a defiant John McCain said that his wife will not release her tax returns, “and neither will my girlfriend.”