Elizabeth Warren, a Harvard professor and former Obama administration official who became a hero to liberals with her sharp critique of the nation’s financial institutions, will announce Wednesday that she is challenging Sen. Scott Brown of Massachusetts for his seat in 2012.

Warren, 62, had been widely expected to run and had been conducting a campaign-style listening tour. She will formally announce her intentions with an online video statement as she travels the state.

“The pressures on middle-class families are worse than ever, but it is the big corporations that get their way in Washington,” she said. “I want to change that. I will work my heart out to earn the trust of the people of Massachusetts.”

Her candidacy will set up one of the nation’s most high-profile Senate races. Democrats, who are trying to hold control of the Senate, hunger for a win in one of the nation’s most liberal states, particularly if it would mean snatching back the seat once held by the late Edward M. Kennedy.

Brown (R) won the seat in a 2010 special election after Kennedy’s death.

But, with an eye toward winning a full six-year term in 2012, he has pursued a shrewd, moderate course in the Senate and has remained popular in the state.

Brown did not immediately respond to Warren’s news.

To challenge Brown, Warren would first have to defeat a crowded field for the Democratic nomination, including CityYear founder Alan Khazei, Newton Mayor Setti Warren and activist Bob Massie.

But Elizabeth Warren is the best-known of Brown’s potential challengers, and a recent WBUR poll showed her nine percentage points behind the senator — closer than any other Democrat.

Warren gained prominence when she was appointed chairman of a 2008 congressional panel charged with overseeing the Troubled Assets Relief Program. In that role, she became an advocate for tighter regulations of banks and stronger protections for mortgage holders, credit card users and other consumers.

President Obama selected her to help guide the creation of the Consumer Financial Protection Bureau, a government agency that began under the Dodd-Frank Wall Street Reform and Consumer Protection Act.

She left the administration and began to lay the groundwork for her Senate campaign after it became clear that congressional Republicans would block her nomination to head the agency. Obama tapped former Ohio attorney general Richard Cordray instead.