Hillary Clinton joins 2008 presidential race

"I welcome her and all the candidates, not as competitors, but as allies in the work of getting our country back on track," he said.

Obama's candidacy has stoked enthusiasm among Democrats looking for an alternative to Clinton, who some fear could be too polarizing to defeat a Republican candidate next year.

Clinton enjoys the political benefits of her husband who, despite his scandal-hit presidency, remains enormously popular and is a powerful fund-raiser, experts noted.

"She comes in with Bill Clinton. She comes in with celebrity. She comes in with money. She comes in with name recognition. She comes in with a built-in following," said Lee Miringoff, pollster with the Marist Institute for Public Opinion in Poughkeepsie, New York. "That's an awful lot."

But some Democrats object to her stance on the U.S.-led war in Iraq, which she voted to authorize in the Senate.

She has since criticized the way the war has been run and said recently she would introduce legislation capping U.S. troops in Iraq at the level before Bush's planned increase of 21,500 additional U.S. troops in that country.

On her Web site, Clinton said she would launch a series of live, online video conversations with voters, on Monday.

The Internet's political influence has grown since Democrat Howard Dean rose from obscurity to temporary front-runner in the 2004 presidential primaries, due largely to contributions and support organized online.

Last week, commentator Arianna Huffington, whose Huffington Post Web site is one of the most widely read U.S. political blogs, invited candidates in the 2008 race to hold what would be the first online debate among presidential contenders.

(Additional reporting by Doina Chiacu in Washington and Carey Gillam in Topeka, Kansas)