Barack Obama has dismissed the idea of becoming Hillary Clinton's running mate in the US presidential election.
Obama is the favourite to win the Mississippi primary
In recent days the Clinton campaign has repeatedly suggested a "dream ticket" combining the two Democratic Party candidates might be a possibility.
But Mr Obama said the proposal made no sense because he was ahead in the race.
Mrs Clinton and Mr Obama are gearing up for Tuesday's primary in Mississippi, the latest in the battle for the Democratic party nomination.
Polls suggest Mr Obama is leading in the state. At stake are 33 delegates to the August convention where the party will choose its candidate for the White House.
"I don't know how somebody who is in second place is offering the vice presidency to somebody who is in first place," he told a rally in Columbus, Mississippi.
"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'm not running for vice president. I am running for president of the United States of America," he added. "I am running to be commander-in-chief."
In a challenge to suggestions from the Clinton campaign that he does not have enough foreign policy experience to be president, he asked: "If I'm not ready, how is it that you think I would be such a great vice president?"
Hillary Clinton was campaigning in Pennsylvania
But Howard Wolfson, a spokesman for Mrs Clinton, defended her campaign's position:
"The answer to that is that Senator Clinton will not choose any candidate who has not at the time of choosing passed the national security threshold. Period.
"But we have a long way to go between now and Denver [where the party convention will be held], and it is not something that she would rule out at this point," he said.
Mrs Clinton headed to Pennsylvania, reflecting the state's importance as the rivals' next major battleground.
The state is due to vote on 22 April with 158 delegates up for grabs.
"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," she said.
Meanwhile John McCain, the presumptive Republican nominee, had a medical check-up and declared to reporters that he was free of cancer.
The Arizona senator has face questions about his health after a bout of skin cancer in 2000.
This week, Mr McCain has embarked on a nationwide fundraising drive.
The Arizona senator flew to St Louis, Missouri, on Monday. Further events were scheduled in New York on Tuesday, Boston on Wednesday and other unannounced cities through the week.
Mr McCain, who is set to secure the Republican nomination at the party's national convention in September after winning the backing of a majority of delegates, is under pressure to build up a campaign war-chest ahead of November's election.
His Democratic rivals have been attracting record donations, meaning the eventual nominee is likely to have plenty to spend on running against him.
Mr Obama goes into the Mississippi primary vote buoyed by a win in caucuses held in Wyoming at the weekend, which gave him seven delegates to Mrs Clinton's five.
DEMOCRATIC DELEGATE RACE
BARACK OBAMA: 1,578
Delegates won on 8 March: 7
States won: 25
HILLARY CLINTON: 1,468
Delegates won on 8 March: 5
States won: 16
Delegates needed to secure nomination: 2,025.
Source: AP at 0015 GMT 9 March
The Illinois senator is expected to do well in the southern state, where black voters - who have heavily supported him in previous primaries - make up a majority of Democratic voters.
Mr Obama currently has a total of 1,578 delegates against 1,468 for his rival, according to the Associated Press news agency. It takes 2,025 to secure the party's nomination.
At campaign stops in Mississippi on Friday, Mrs Clinton focused on the need for greater urgency in rebuilding communities devastated by Hurricane Katrina in 2005.
Former President Bill Clinton also campaigned in the state at the weekend, when he argued that a joint presidential bid with his wife heading the ticket and Mr Obama as her vice-president "would be an almost unstoppable force".