Democratic candidates for governor Adam Cote and Janet Mills (Carl D. Walsh/Portland Press Herald)

A national political organization dedicated to electing Democratic women contributed $300,000 to support Attorney General Janet Mills’ gubernatorial bid and oppose challenger Adam Cote.

The $300,000 donation from EMILY’s List is the first major infusion of “outside money” into Maine’s Democratic primary race, and comes days before the June 12 primary.

The South Portland-based political action committee that received the donation, Maine Women Together, disclosed spending $192,500 on Tuesday to oppose Cote and $7,500 to support Mills.

Outside groups are prohibited by law from coordinating with campaigns when making independent expenditures. Cote’s campaign hit back Wednesday, however, portraying the spending as an “out-of-state bailout of Mills’ campaign.”

“This is outrageous and Maine people shouldn’t stand for it,” Cote, a veteran and lawyer from Sanford, said in a statement. “To have an outside group, no matter who it is, drop $200,000 into a Maine gubernatorial race to attack one candidate, six days before an election, is unprecedented and exactly what people hate about politics.”

Mills and Cote have been trading political jabs with increasing frequency, including during their final debate Tuesday when the two had a brief-but-pointed exchange over guns and the National Rifle Association. The two perceived front-runners have also referred to each other in television ads.

Until a few weeks ago, the crowded Democratic primary had been a largely cordial race due, in large part, to concerns about how “going negative” could affect a campaign’s standing with voters during Maine’s first-ever use of ranked-choice voting.

Maine Women Together, a pro-Mills group created by former state lawmaker Lynn Bromley of South Portland and Bonita Usher of Buxton, indicated the $200,000 would pay for video and web ads as well as social media promotion by H Street Strategy, a Washington, D.C.-based political consulting firm.

The other Democrats in the seven-person primary are former Biddeford Mayor Donna Dion, Portland Sen. Mark Dion, former House Speaker Mark Eves of North Berwick, former Portland Rep. Diane Russell and advocate Betsy Sweet of Hallowell.

So-called independent expenditures are less common in primaries but have become commonplace in Maine elections. During the 2014 gubernatorial elections, for instance, groups operating independently of the campaigns spent millions of dollars either for or against Republican Gov. Paul LePage and his Democratic challenger, U.S. Rep. Mike Michaud.

The Moose Tracks PAC is the only independent expenditure reported so far in the four-person Republican primary, although that could change as June 12 gets closer.

The principal officer of Moose Tracks is Paul Coulombe, a Southport resident and liquor entrepreneur who is a close ally of LePage. The radio ads aired last week by Moose Tracks criticize Gorham businessman Shawn Moody while touting Mayhew.

The other two Republican candidates in the race are state Senate Majority Leader Garrett Mason of Lisbon Falls and House Minority Leader Kenneth Fredette of Newport.

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