Updated 9:38 a.m. ET

Consumer advocate Elizabeth Warren launched her campaign for the U.S. Senate against Republican Scott Brown as she greeted Boston commuters this morning before touring the state.

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

Brown, who is up for re-election in 2012, is one of the most vulnerable Republicans in the Senate. He won an upset victory in a special election to succeed the late Edward Kennedy in heavily Democratic Massachusetts.

Warren, a Harvard law professor, helped President Obama launch the Consumer Protection Financial Bureau, which was created by a new law that set up rules for Wall Street.

She left the Obama administration in July when Obama nominated Richard Cordray to the lead the agency. Warren set up an exploratory committee for the Senate race last month and has been meeting with small groups of voters across the Bay State.

Several Democrats have filed for the race against Brown, who has amassed more than $10 million in his campaign account in anticipation of a competitive race. Alan Khazei, founder of City Year, has led Democrats in the fundraising race for the primary.

The non-partisan Cook Political Report rated the Massachusetts Senate race a tossup, meaning either party has a chance to win.

Note: This post was first published Tuesday, Sept. 13 and updated to reflect Warren's entry into the race.