Blumenthal announced his decision to run Wednesday at Democratic headquarters in Hartford, just hours after Dodd announced from his East Haddam home that he would not be seeking a sixth term in November.
Dodd has been dogged by lagging poll numbers for months.
Blumenthal has been Connecticut's attorney general since 1990. He has gained a national profile by challenging Big Tobacco and its alleged marketing to children and the Web site Craigslist and its erotic ads that promote prostitution.
Recent polls show the 63-year-old Blumenthal is one of the most popular politicians in Connecticut.