Elizabeth Warren laughed at the notion of running for Senate when Rep. Barney Frank (D-Mass.) broached the idea early this year.

At the time, the fiery Harvard law professor was caught in a political stalemate over whether she should lead the new federal consumer watchdog agency that she is widely credited with inventing. Republicans reviled her, and she had never run for elected office.

“I just kind of fluffed it off,” Warren said in an interview. “It’s just one of those things you smile at and move to the next topic.”

Yet now, Warren has emerged as one of the linchpins in Democrats’ strategy to maintain control of the Senate in next year’s elections: Last week, she entered a Massachusetts race that is among her party’s best hopes for stealing back a seat from Republicans.

To win, Warren must transform herself from a wonky regulator to a homespun candidate who can hold court with voters in a state famous for its rough-and-tumble politics and local pride. She officially launched her campaign last Wednesday with a whirlwind two-day tour of the state, thronged by reporters and photographers — including one of her own hired cameras.

“How are you holding up?” Warren’s photographer asked as he directed her to order a coffee at a Dunkin’ Donuts in South Boston.

“Are you kidding? This is fun!” she said.

“You think this is fun?” he asked as he snapped pictures of her shaking hands with workers behind the counter.

“It’s better than a congressional hearing,” Warren quipped.

A key race for Democrats

Losing the Senate seat in Massachusetts was one of the most deflating defeats for Democrats in 2010 amid a widespread rout that left them with tenuous control over the chamber. Next year, they will face tough races to protect seats in Virginia and Missouri that could make it difficult for the party to hold on to the majority.

Massachusetts is one of the few states where Democrats could gain a seat — one that was held for four decades by Edward M. Kennedy until his death in 2009.

“The stakes are very, very high for Democrats nationally,” said Phil Johnston, a Democratic strategist and veteran of Massachusetts politics. “We don’t like seeing Ted Kennedy’s seat going to a conservative Republican.”

On the first day of her campaign, Warren delivered a message of fighting against established interests in Washington and remedying the economic distress of the middle class. Her stump speech called for closing corporate tax loopholes, creating jobs through spending on infrastructure and investing in education. But she hit her stride when launching into more familiar territory: the broken financial system that puts consumers — or, in this case, middle-class Massachusetts residents — at a disadvantage.

“This is not change for me,” she said. “This is what I’ve worked on all my life.”

Warren landed on the political stage in the aftermath of the financial crisis when President Obama named her to the congressional panel overseeing the government’s bailout of the financial system. She had spent decades researching the impact of debt on American families, publishing prescient papers such as “Mortgage Debt, Bankruptcy and the Sustainability of Home Ownership” in 2005.When she arrived in Washington, she quickly made waves for her antagonism toward Wall Street and her public needling of Treasury Secretary Timothy F. Geithner over bailouts for big banks.

That stance made her a champion among Democrats but incensed Republicans. Warren became a polarizing figure, torpedoing her chances of landing her dream job: leading the consumer watchdog agency that she helped create.

In July, Obama named former Ohio attorney general Richard Cordray to the post instead. Within hours, Warren responded by accusing the GOP of attempting to weaken the Consumer Financial Protection Bureau (CFPB) and floated the possibility of running for the Senate.

“I’m saving all the rocks in my pockets for the Republicans,” she told liberal bloggers at the time.

But in her campaign now, Warren has backed away from such blatant partisan rhetoric as she faces an uphill climb to beat a popular Republican incumbent. Though Massachusetts leans left, only 36 percent of voters are registered as Democrats, while 11 percent are Republican. Just over half of voters are officially declared independents, and the state’s open primary makes them a particularly influential voting bloc. In an interview, Warren said she meant only to indicate that she is an outsider in Washington who is ready to fight.

“I’m not running for Senate so I can be the 100th-in-seniority, be-polite-and-make-no-difference senator from Massachusetts,” she said.

A Red Sox test

Outside the J&M Diner in Framingham, Mass., Warren fielded questions from local reporters on issues ranging from Obama’s jobs bill to Gov. Deval L. Patrick’s safe-communities initiative. Then came the tricky question, on a topic that famously helped fell the last Democratic challenger for the seat: What did she think about the Red Sox game the previous night?

“Those five innings, I was getting nervous,” Warren said of the Sox’ late scoring in a victory over the Toronto Blue Jays.

Republicans — and even some Democrats — have criticized Warren as an untested politician with a Harvard connection who is unfamiliar with the Massachusetts grass roots. The primary election alone will pit her against a crowded field of challengers with strong local ties.

There’s Newton Mayor Setti Warren, who worked for Sen. John F. Kerry (D-Mass.) and is an Iraq war veteran. Bob Massie, an Episcopal priest who was infected with HIV through a blood transfusion, has enlisted longtime Democratic campaign strategist Joe Trippi to his side. Social entrepreneur Alan Khazei, who co-founded a popular nonprofit organization, has raised nearly $1 million.

And then there is Scott Brown, the senator with sky-high approval ratings whom Warren hopes to unseat. Brown has carefully honed his image as Massachusetts’ working man, campaigning in a beat-up pickup truck and recently serving a seven-day tour in Afghanistan with the National Guard. More than half of voters said they viewed him favorably in a poll this month conducted by local radio station WBUR. Just 17 percent viewed Warren positively, though that figure was higher than that for other Democrats. And while more voters said they had heard of her than the other candidates, 44 percent still do not know who she is, the poll showed.

“She’s not going to be able to run the listening tour, Rose Garden campaign that she started with,” said Tobe Berkovitz, a professor of communication at Boston University. “Is she as good a retail politician as advertised?”

Warren has hired veteran Massachusetts politico Doug Rubin to mastermind her race. To introduce Warren to voters before her official announcement, Rubin turned to the expansive voter networks developed when he helped get Deval Patrick elected governor in 2006 and 2010. At several campaign stops, residents seemed as eager to catch a glimpse of him as they did of Warren.

On the trail, Warren peppered her speeches with personal anecdotes designed to dispel characterizations of her as a Harvard elitist. There’s the story of her father’s heart attack that upended the family’s finances; her first job, at age 9, looking after a colicky baby; and the time her brother sent a pig on a motorcycle down her local high school’s hallway. She emphasizes her early career as a special-education teacher at a public elementary school, rather than the class she teaches now at Harvard.

“I work at Harvard, but I was not born at Harvard,” Warren said.

The money race

The appeal for money went out on Day 2 of the campaign.

“We’ve got to make sure this campaign has enough resources, right from the start, to fight back against the big corporations,” Warren wrote in an e-mail to supporters.

The campaign set a goal of 10,000 donations of at least $5 within the first 100 hours. It surpassed it in one day. The liberal Progressive Change Campaign Committee, which began calling for Warren to run the day that she was passed over to head the CFPB, has raised more than $300,000 for her.

The race is sure to be an expensive one. Primary season in Massachusetts will last through next September, leaving only six weeks to campaign for the general election. Meanwhile, Brown has amassed a nearly $10 million campaign treasury.

Warren’s staff members declined to say how much she has received so far. But raising money could become a minefield given her strong rhetoric against large financial institutions and corporations, which can also deliver big donations. Warren did not directly respond when asked in an interview whether she would accept money from those groups.

“Nobody’s gonna doubt where I stand on this set of issues,” she said.

But leading Democratic fundraiser Khazei has gone on the attack, challenging Warren also to reject money from lobbyists and political action committees. Her staff members responded only that her record was clear.

Jim Clark, 64, a retiree from Worcester, Mass., considers himself an independent and has never been politically active. But he saw Warren on TV talking about the problems with the nation’s financial system and was impressed.

“Anybody that the banks hate has gotta be okay with me,” he said. “I’m coming out for her.”

He didn’t know how to donate except by showing up for her campaign stop last week at the Parkway Diner in his home town. He brought a folded check for $25, made out to the Committee to Elect Elizabeth Warren. He guessed that name sounded right.

Clark stood at the bar as Warren worked the room and was one of the last people to meet her.

“Senator,” he called her as he shook her hand. Then he pulled the check out of his pocket and presented it to her. Warren looked at it and smiled.

“This means a lot,” she said.