Elizabeth Warren, a Harvard law professor and former Obama administration official, announced Wednesday morning via Web video that she will challenge Republican Sen. Scott Brown for his seat in Massachusetts.

Warren began her first day of campaigning at a subway stop in South Boston — an area filled with the kind of conservative Democrats that Warren, a 62-year-old Oklahoma native,must win over.

At stops in southern and central Massachusetts, she repeatedly invoked her early career as a special-education teacher in public schools and her working-class childhood to counter criticism of her time at Harvard. She also touted her connections to Massachusetts — her husband is a native and her nephew attends a private college in Massachusetts.

“I’m not so sure if this is about geography or about heart,” she said.

In her announcement video, Brown is not named — and neither is Warren. Instead, she focuses intently on the middle class, which she said “has been chipped at, hacked at, squeezed and hammered for a generation now.”

Although she has never run for office, Warren has built up a national group of liberal fans because of her work on Capitol Hill. She was chosen by Democrats to chair Congress’s panel overseeing federal bailout spending, and President Obama picked her to help create the Consumer Financial Protection Bureau.

The liberal Progressive Change Campaign Committee said it has received $200,000 in contributions in support of Warren.

She was expected to lead the consumer bureau, which was created last summer. But Republicans in the Senate blocked her nomination.

In Boston, Warren often invoked her controversial role at the congressional oversight panel and the Consumer Financial Protection Bureau as a badge of honor.

“I’m willing to throw my body in front of the bus to stop bad ideas” in Washington, she told a passerby.

Warren will not have an easy path to the nomination; she faces CityYear founder Alan Khazei, activist Bob Massie and Newton Mayor Setti Warren in the primary. Khazei and Warren are expected to put up a tough fight in what is likely to be a competitive race.

Warren, however, can point to a recent poll that showed she would fare best against Brown, who has remained popular in the state by charting a moderate course on hot-button issues.

“The early questions were mainly about her political instincts and talents,” Democratic strategist Jim Jordan said. “The early indicators in those regards are very, very good. Her message and performance and the character she’s projected have been absolutely first-rate.”

Warren and her staff deflected questions on the trail about her ability to raise enough money to compete with Brown’s $10 million war chest.

“I can be outspent but I can’t be outworked,” she said.

Brown has not directly attacked Warren, but Brian Walsh, the National Republican Senatorial Committee’s communications director, called her “a tax-and-spend liberal from Harvard.”

Brown won the Senate seat previously held by the late Edward M. Kennedy (D) in a special election last year, a surprising upset. His Democratic opponent in that race, Massachusetts Attorney General Martha Coakley, was widely pilloried for her lackluster campaign style.

Warren seems determined to not make the same mistake.

“How you holding up?” a campaign photographer asked at one point.

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

“You think this is fun?” he said.

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