Graham Platner, Mills’s Republican primary opponent, draws support from progressive figures like Bernie Sanders and Elizabeth Warren, has outpaced Mills in fundraising and led some polls, while a third candidate, David Costello, is also in the race.

GOP messaging highlights Collins’s role as chair of the Senate Appropriations Committee, contrasting her with Democratic challengers; One Nation runs positive ads for Collins, and Pine Tree Results—tied to Leonard Leo’s network—placed a June ad reservation.

A large Republican fundraising push has formed to defend Susan Collins in Maine’s U.S. Senate race, backed by early ad reservations and heavy outside spending.

Mills’s campaign has attacked Platner over past Reddit posts on sexual violence; Platner apologized for the comments, and he previously faced scrutiny over a chest tattoo that he chose to cover.

Democratic groups are expected to accelerate spending after the Maine primary to counter Republican advantages as national attention and outside money from both sides surge.

Collins, despite a rocky relationship with President Trump, is viewed as crucial to Republican control of the Senate, making Maine a key national battleground.

Republican groups, including the Senate Leadership Fund and One Nation, have reserved roughly $57 million in ads through Election Day, with SLF alone planning at least $42 million for ads and GOTV.

Analysts anticipate sustained outside spending and a potential reshaping of national campaign strategies after Maine’s primary and other state races, signaling a historic level of fundraising and ad spending akin to prior large campaigns.