BRISTOW -- White House hopeful Barack Obama paid tribute Thursday to his defeated Democratic rival Hillary Clinton, seeking to mend fences as he turned his sights on Republican candidate John McCain.

Fresh from securing the Democratic nomination, Obama ventured into the usually solidly Republican state of Virginia vowing to unite his party for the November election after his close and sometimes bitter battle against Clinton.

"Senator Clinton, she made history," Obama told a jubilant crowd at a 25,000-seat concert pavilion in Bristow.

"There aren't many in public life in our history who is smarter, more diligent, more dedicated to the cause of lifting up those in need than Senator Clinton," he said.

"I am a better candidate because of the work she did, and she deserves ... our respect and our gratitude," he said. "And my two daughters see themselves differently because she ran for president of the United States of America."

Some Democrats have floated the idea of a "dream ticket" pairing Obama as president and Clinton as vice president.

But the former first lady, who prepared to end her campaign Saturday and throw her full support behind Obama, denied that she was agitating for the number two spot.

"The choice here is Senator Obama's and his alone," a statement from the Clinton campaign said, without mentioning if she would accept the job if it were offered to her.

Obama did not talk about the vice presidential spot during his speech, but he lavishly praised two Democrats seen as possible running mates, Virginia's governor Tim Kaine and Senator Jim Webb, who introduced him at the rally.

He also assured his supporters that the party would be united to take the White House from the Republicans after eight years under President George W. Bush.

"I know there are those who say this primary race has left us divided. I say they're wrong," Obama told the crowd.

"I know we won't be divided because whatever differences between me and Hillary Clinton, they pale in comparison to the differences we have with the other side," he said.