First-time candidate Elizabeth Warren has eclipsed her five Democratic rivals and emerged as the leading challenger to Massachusetts Republican Sen. Scott Brown just three weeks after launching her campaign.

Warren, who has a national following after spending nearly two decades as a consumer advocate and Harvard law professor, is largely untested on the campaign trail. So her fast start has relieved and energized many Democrats who had been clamoring for a major challenger for months in their hopes to reclaim the seat once held by Sen. Edward Kennedy.

"It is a long, grueling and very public process, but so far she has handled herself well," said Mike Shea, a veteran Democratic consultant. "What Democrats want more than anything is a winner. And she looks like a winner."

Democrats face a difficult fight to unseat Brown. He is a strong fundraiser, with nearly $10 million at the end of June. And he's popular among independents who have strong sway in Massachusetts races.

Warren turned in a solid performance and faced no direct jabs from her rivals in the race's opening debate Tuesday night. And she is far outpacing the other Democratic candidates _ they poll in single digits while she polled at 38 percent, about even with Brown in a recent poll. National progressive groups are providing fundraising and organizational muscle. A video clip of Warren calling for more taxes on the rich fired up her liberal base of support and became a YouTube hit.

Newton Mayor Setti Warren, no relation, cited her surging candidacy as a major reason for dropping out of the race last week.

"Elizabeth Warren has captured the imagination of Democrats nationally, here in the state," he said. "She changed the dynamics of the race certainly for me and it was clear to me that because of that I was not going to be able to go on and win."

But another Democratic candidate, Alan Khazei, said that even though party leaders in Washington are behind her and she's ahead in polls, the race is far from over.

"She's made a big splash getting into the race," said Khazei, known for his fundraising and grassroots organizing abilities as City Year youth program co-founder. "She's gotten a lot of support from the Washington establishment and they can bring a lot of attention. But this election is just getting started. Polls a year out are meaningless."

Khazei said no one knows where Warren stands on many key issues.

"We have to see what kind of agenda she has," he said.

She's struck populist themes, casting herself as a defender of middle-class families against wealthy corporate and financial special interests.

The YouTube video of Warren justifying more taxes for the rich went viral, but it also sparked sharp criticism from Republicans who accused her of waging class warfare and suggested she was being un-American, bolstering their broader line of attack on Warren as an out-of-touch Harvard liberal. The video had more than 632,000 views.

"Elizabeth Warren and her inflammatory rhetoric will divide our country and our Commonwealth at a time when we need to come together to confront the very serious economic challenges facing us,'" Jim Barnett, Brown's campaign manager, said in a fundraising email. "Let's remember we're Americans first."

Shea said the GOP has trained its fire on Warren because they see her as Brown's biggest threat.

Warren has won support from national groups such as EMILY's List, which raises money for female Democratic candidates, and the Progressive Change Campaign Committee, a national liberal group which has raised more than $400,000 for her. MoveOn.org, a progressive group, said it raised more than $300,000 for Warren in less than 24 hours.

She'll need that support to help cut into Brown's money advantage.

Faced with a crowded field, Democrats worry that a long, costly and divisive primary could dash their hopes of reclaiming the seat after their embarrassing loss to Brown in 2010. Many party officials hope the party can unite behind Warren, the sooner the better.

Rep. Niki Tsongas, the only woman in the Massachusetts congressional delegation, endorsed Warren this week.

Warren was chosen by President Barack Obama last year to set up a new consumer protection agency, but congressional Republicans opposed her becoming the director.

Other Democrats running for the seat are attorney Marisa DeFranco, state Rep. Tom Conroy, Newton resident Herb Robinson and Robert Massie, who unsuccessfully ran for lieutenant governor in 1994.