Obama, campaigning in Mississippi ahead of the state's contest on Tuesday, said he has won more states than Clinton and is leading in delegates who will decide the Democratic candidate to face Republican John McCain in November.

"I don't know how somebody who is in second place is offering the vice presidency to somebody who is in first place," Obama, an Illinois senator, told supporters. The crowd booed when he mentioned Hillary's idea.

"I'm not running for vice president. I am running for president of the United States of America," Obama added. "I am running to be commander in chief."

Obama said the New York senator and former first lady was playing a political game, denigrating his abilities at the same time she promoted the idea of placing him in a secondary role.

"I do not believe Senator Clinton is about change because in fact this kind of gamesmanship - talking about me as vice president, but maybe he's not ready for commander in chief - that's exactly the kind of double-speak, double-talk that Washington is very good at," he said.

Clinton has raised the idea of a joint ticket with Obama several times since she saved her campaign with big wins in three of four contests last week, including the states of Ohio and Texas.

Her suggestions are aimed at paving the way for the idea of putting her at the top of the ticket even though she trails Obama in the close race, analysts said, while building a bridge to his sizable bloc of supporters.

Clinton has hammered Obama as unready for the Oval Office and too inexperienced to handle the task of commander in chief. Her campaign on Monday lined up former military officers to question Obama's national security credentials.

Obama took note of Clinton's repeated attacks and said the vice president's primary role would be to take over if the president died or was incapacitated.

"If I'm not ready, how is it that you think I would be such a great vice president? Do you understand that?" he asked.

Asked about the contradiction of touting Obama as a vice presidential candidate while condemning his ability to lead, Clinton spokesman Howard Wolfson implied there was still time for Obama to prove himself before the Democratic Party convention in Denver in August.

'LONG WAY TO GO'

"We do not believe Senator Obama has passed the commander in chief test," Wolfson said. "But there is a long way to go between now and Denver."

Obama told reporters he decided he needed to address the joint ticket issue because Clinton and her husband, former President Bill Clinton, kept bringing it up.

"The Clintons have spent all weekend talking about it, so I wanted to make sure there wasn't any ambiguity about it," he said during a stop at a Columbus diner.

Obama, who would be the first black U.S. president, is heavily favored in Tuesday's Mississippi contest, the next showdown in his back-and-forth duel with Clinton.

Black voters, who have backed Obama heavily, may account for more than half the Democratic primary voters on Tuesday.

Obama hopes to add to his almost insurmountable lead in pledged delegates who will help decide the nominee. Mississippi has 33 pledged delegates at stake.

Neither Obama nor Clinton is likely to reach the 2,025 delegates needed to clinch the nomination without help from nearly 800 "superdelegates" - party officials and insiders free to back any candidate.

The states of Michigan and Florida, which were stripped of their delegates in a dispute with the national party and held unsanctioned contests, also could figure in a final resolution to the tight race.

Officials in both states have discussed redoing their contests so they would produce delegates to the convention, but the candidates, the state parties and national party would have to agree on the timing, funding and formats.

State and national party leaders have shown interest redoing the votes by mail, a less expensive option.