Bredesen is the last Democrat to win a statewide election in Tennessee, a state that famously turned its back on favorite son Al Gore in the 2000 presidential election in favor of Republican George W. Bush.
Bredesen announced his candidacy in a campaign video posted on his website on Thursday morning. He had spent the previous day calling potential donors to let them know of his plans.
Bredesen grew up in a single-parent, working class home in Shortsville, New York, went to Harvard on an academic scholarship and earned a degree in physics.
He became a wealthy health care entrepreneur before winning two terms each as Nashville mayor and Tennessee governor.