The Maine governor said on Thursday she lacked the money to continue, leaving progressive oyster farmer Graham Platner poised to challenge Republican Senator Susan Collins.
Mills, once national Democrats' preferred candidate, quit as Platner kept leading in polls and fundraising in a race seen as one of the year's most important Senate contests.
Her withdrawal reshapes Democrats' effort to unseat five-term incumbent Collins, whom they still hope to beat amid broader backlash against Republicans and President Donald Trump.
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