Gov. Janet Mills suspended her campaign for the U.S. Senate Thursday, all but ensuring progressive oyster farmer Graham Platner will face Republican Senator Susan Collins in November. The announcement marks a stunning denouement for the two-term governor and former attorney general and district attorney in the state. Mills was recruited by national Democrats to run and had the backing of Senate leaders, i ncluding Minority Leader Chuck Schumer . For months, Mills had been telling voters she was the more electable candidate against Collins, who has held the seat since 1997 and defied tough political headwinds before . But Mills had recently pulled down her TV advertisement buys, and polling of the race showed her trailing Platner by as many as 30 points ahead of the June 9 primary. Mills did not mention Platner or the general election race in the announcement suspending her campaign, blaming her exit on a lack of financial resources she'd need to continue.…