COLUMBUS, Miss. — Senator Obama ridiculed the idea of being Senator Clinton’s running mate today, saying voters must choose between the two for the top spot on the fall ticket.

The Illinois senator used his first public appearance of the week to knock down the notion that he might accept the party’s vice presidential nomination. He noted that he has won more states, votes and delegates than Mrs. Clinton so far.

“I don’t know how somebody who is in second place is offering the vice presidency to the person who is first place,” Mr. Obama said, drawing cheers and a long standing ovation from about 1,700 people in Columbus, Miss.

Saying he wanted to be “absolutely clear,” he added: “I don’t want anybody here thinking that somehow, ‘Well, you know, maybe I can get both.’ Don’t think that way. You have to make a choice in this election.”

“I am not running for vice president,” Mr. Obama said. “I am running for president of the United States of America.”

Mr. Obama aides said Mrs. Clinton’s recent hints that she might welcome him as her vice presidential candidate appeared meant to diminish him and to attract undecided voters in the remaining primary states by suggesting they can have a “dream ticket.”

Mr. Obama had never suggested he might accept a second spot on the ticket. But until today he had not ridiculed the notion so directly, even if he did completely rule it out in Shermanesque terms.

He told the audience that it made no sense for Mrs. Clinton to suggest he is not ready to be president and then hint that she might hand him the job that could make him president at a moment’s notice.

“If I’m not ready, how is it that you think I should be such a great vice president?” he said, as the crowd laughed and cheered loudly.

Mississippi holds it primary Tuesday, the last contest before the Pennsylvania primary six weeks from now.

Mrs. Clinton and her husband, the former president, had suggested recently that a Clinton-Obama ticket would be popular and formidable against Senator McCain in November.

“A lot of Democrats like us both and have been very hopeful that they wouldn’t have to make a choice but obviously Democrats have to make a choice and I’m looking forward to getting the nomination,” Mrs. Clinton said today in Scranton, Pa. “And it’s preliminary to talk about whoever might be on whose ticket.”

Many political activists discounted the notion all along. They noted that the two senators lack a warm relationship and, more important, that Mr. Obama would be ill-served by hinting he might accept the vice presidential slot when he holds the lead in delegates and hopes to win the presidential nomination.

In the latest Associated Press count, Mr. Obama leads Mrs. Clinton, 1,579-1,473. He has won 28 contests to her 17.

Moreover, many insiders feel the ambitious and fast-rising Mr. Obama would chafe in the vice president’s job, especially in a White House where Mrs. Clinton’s husband would almost surely play a huge advisory role.

Still, the notion of a Clinton-Obama ticket has received ample discussion in recent days on cable TV news shows and newspapers such as New York City’s tabloids.

In an interview Friday in Wyoming with KTVQ-TV, a CBS affiliate based in Billings, Mont., Mr. Obama’s comments were somewhat mixed.

“Well, you know, I think it’s premature,” he said of accepting the second spot on the ticket. “You won’t see me as a vice presidential candidate.”

His remarks today were more detailed, pointed, and humorous.

Of course, they will not completely end the speculation. Presidential candidates routinely disavow any interest in the vice presidential spot. But some, including John Edwards and Al Gore, change their minds when they fall short of their top goal.