Flanagan defeated centrist U.S. Rep. Angie Craig with a pledge to fight the influence of corporations and confront Republican President Donald Trump.
She was backed by popular progressives including Sens. Bernie Sanders and Elizabeth Warren, who rallied with her in the campaign's final weeks. She was also backed by the retiring incumbent, Democratic Sen. Tina Smith.
Flanagan highlighted Craig's support from wealthy donors and blasted her vote for a Trump-backed immigration bill, which became a liability after immigration agents shot and killed Renee Good and Alex Pretti in Minneapolis.
Flanagan enters the general election as the frontrunner in a state that hasn't elected a Republican to a statewide office in 20 years. But Republicans see an opening from Flanagan's progressive views and backlash to a fraud scandal involving the widespread theft of public funds dispersed by state agencies to social services organizations. Republicans say Democrats turned a blind eye while Flanagan was lieutenant governor.