A doctor who ran against Mitt Romney for Massachusetts governor a decade ago won the chance to challenge him again, this time as the Green Party's presidential nominee.

Jill Stein, 62, an internist from Lexington, Massachusetts, criticised both Mr Romney and President Barack Obama, saying both had become too dependent on donations from corporations in order to acquire office at the expense of the nation's citizens.

"We need real public servants who listen to the people - not to the corporate lobbyists that funnel campaign checks into the big war chests," Ms Stein told applauding supporters at a Holiday Inn in Baltimore.

"That's what brought me to the Green Party, the only national party that is not bought and paid for by corporate money."

Ms Stein acknowledges that her candidacy is a very long shot. Still, she notes that a growing number of people are expressing frustration with the two major political parties and she cites the Occupy Wall Street movement as an example of that.

Ms Stein won 193.5 delegates, compared to 72 for comedian Roseanne Barr, who did not attend.