Obama made the remarks at a rally in Columbus, Mississippi, CNN reported.

Clinton, Obama's rival for the Democratic presidential nomination, and her husband former President Bill Clinton have suggested a joint Clinton-Obama ticket -- with Obama in the second slot. Obama forcefully shot that idea down.

"Senator Clinton is fighting hard. She's tenacious. I respect her for that. She is working hard to win the nomination. But I want everybody to be absolutely clear. I'm not running for vice president. I'm running for president of the United States of America," Obama told supporters.

"With all due respect. I won twice as many states as Senator Clinton. I've won more of the popular vote than Senator Clinton. I have more delegates than Senator Clinton. So, I don't know how somebody who's in second place is offering vice presidency to the person who's in first place," he said.

Obama was campaigning in Mississippi a day before the state holds its primary today, while Clinton keeps her eye on the next big prize on the Democratic calendar, Pennsylvania.

Thirty-three delegates are at stake today, and in the tight Democratic race, every delegate is critical.

According to CNN's tally, Obama now leads Clinton 1,553 to 1,438 in number of the delegates they have.

A candidate needs 2,025 delegates to win the Democratic presidential nomination.

Obama won the Wyoming caucuses last Saturday, where he picked up seven delegates. Clinton won five delegates.