U.S. presidential hopeful Barack Obama ridiculed the idea of being Hillary Clinton's running mate Monday, saying voters must choose between the two Democrat rivals for the top spot on the fall ticket.

Barack Obama shakes hands with supporters as he enters a town hall meeting at the Mississippi University for Women in Columbus, Miss., on Monday.
(Alex Brandon/Associated Press)

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 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," Obama said, drawing cheers and a long standing ovation from about 1,700 people in Columbus, Miss.

"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," Obama added. "I am running for president of the United States of America."

Obama aides said 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."

Obama told the audience that it made no sense for 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 primary on Tuesday

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

Clinton and her husband, former U.S. president Bill Clinton, have suggested that a Clinton-Obama ticket would be popular and formidable against Republican Senator John McCain in the November general election in the United States.

"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," Clinton said Monday in Scranton, Pa.

"And it's preliminary to talk about whoever might be on whose ticket."

Many political analysts discounted the notion all along, noting that the two senators lack a warm relationship. Analysts also say the ambitious and fast-rising Obama would not want a vice-president's job, especially in a White House where Bill Clinton would likely have a large advisory role.

Analysts also note that Obama would be ill-served by embracing the vice-presidential suggestions when he's leading the race.

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