WASHINGTON  It was the pinnacle moment of his political career. The Senate majority leader, Harry Reid of Nevada, on the verge of making history by shepherding through far-reaching health care legislation, was called upon by the clerk to cast his vote.

And Mr. Reid, who had fought tirelessly for months to get the health care bill adopted, looked up from his desk and said, “No.”

For a millisecond there was confusion in the chamber. Had he lost it? Was he joking? Within half a second, Mr. Reid had switched his vote to “yes.” After 25 straight days of bitter, partisan debate, senators on both sides of the aisle burst out laughing.

“To be honest, I’d like to say I was trying to be funny or create some bipartisanship,” Mr. Reid said later in his office, where he slumped, exhausted into a leather armchair after the vote and a quick news conference. “But I was just in dreamland, thinking about where we had come.”