Maine Governor Janet Mills is seeking the Democratic nomination in the June 9 U.S. Senate primary to challenge incumbent Sen. Susan Collins. Mills, a two-term governor with decades of experience in public office, has been endorsed by Senate Minority Leader Chuck Schumer, the Democratic Senatorial Campaign Committee, EMILY's List and Planned Parenthood Action Fund.

Mills, 78, was Maine's first woman district attorney, first woman attorney general and first woman governor. More than four decades ago, she was the first female prosecutor in the state attorney general's criminal division and secured a successful verdict in a murder trial. She also co-founded the Maine Women's Lobby, which has pushed for gender equity since the 1970s.

"That wasn't the first time someone underestimated me. And it certainly wasn't the last." Mills wrote in a recent memo to campaign donors.

Mills has said she will serve only one term in the Senate if elected. Her top opponent in the primary, Graham Platner, a 41-year-old military veteran and oyster farmer, has raised more campaign money than Mills and has run television and radio ads since entering the race last summer. Platner has held public office no higher than a small-town planning board and has been endorsed by Senator Bernie Sanders. He says voters should elect a senator who will stick around in Washington to gain seniority and influence.

A debate between the two candidates is scheduled for May 7, the first of five Democratic primary debates. Mills has said smaller, more intimate campaign gatherings help her connect with voters, and after a Portland rally in support of Planned Parenthood she contrasted her record with Platner's on reproductive rights.

"He's been nowhere on these issues. He's never walked the walk." Mills said.

Mills has also said Platner carries political baggage that makes him a riskier general-election candidate. Platner has disavowed inflammatory comments he made in old online postings. He has a skull-and-crossbones tattoo recognized as a Nazi symbol, which he has since covered up, and has said he got it during a night of drinking while on military leave in Croatia.

Emily Cain, a former Maine state lawmaker and former executive director of EMILY's List, framed the choice facing primary voters. "Who do you think has the best chance of beating Sen. Collins?" Cain said. "If it's just about who you like better, or who makes you feel better, then that is different than who you think can win in the fall."

"I think the governor has a path to victory," Cain said. "I think it's going to be up to her, her team and her supporters to get across that finish line."