Maine Gov. Janet Mills is expected to launch a campaign for Senate on Tuesday, two Democratic sources familiar with her plans confirmed to CBS News.
A Democrat, Mills is vying to unseat longtime Republican Sen. Susan Collins in one of the most closely watched contests of next year's midterm elections.
Mills' planned campaign launch was first reported by Axios.
Mills, who is term-limited, has served as governor of Maine since 2019, after working as the state's attorney general. She drew national attention earlier this year when she verbally argued with President Trump at a White House event after he pressed Maine to comply with his administration's push to block transgender girls and women from competing in female sports.
"See you in court," Mills said to Mr. Trump.
Mills was a top recruit for the Democratic Senatorial Campaign Committee. She will enter an expanding primary field that includes three other Democratic contenders — among them Graham Platner, an oyster farmer who has emerged as a rising progressive and anti-establishment voice.
The Maine Senate race could mark a rare opportunity next year for Democrats to flip a GOP-held seat and contest the Senate's 53-47 Republican majority, despite an otherwise challenging Senate terrain. Maine leans Democratic, with Joe Biden defeating Mr. Trump by 9 percentage points in 2020 and Kamala Harris beating Mr. Trump by 7 percentage points last year.
But Democrats have struggled for years against Collins, a five-term incumbent with a track record of breaking with Mr. Trump and her congressional Republican colleagues. She won reelection in 2020 by an 8.6-point margin, despite Democratic nominee Sara Gideon spending nearly $62.9 million on the race — more than double Collins' $29.6 million.
