In Wiscasset, Maine, on July 18, 2026, Maine Democrats began the process to select a successor to former nominee Graham Platner. The two-day delegate-selection process involved meetings in eight counties and resulted in 319 of 500 open delegate slots being chosen on Saturday. Jackson-aligned candidates won a majority of the slots, while supporters of Nirav Shah and Shenna Bellows each received a small number of slots. In Cumberland County, Jackson-aligned candidates secured all delegate positions, and the final alternate delegate was selected by drawing names from a baseball cap. Observers view the race as an important opportunity for Senate Democrats, as Republican Susan Collins is the only Republican seeking reelection in a state that Donald Trump lost in 2024. Jackson said the contest is likely the biggest in the country and noted that Collins is a different type of opponent. Jackson's campaign mobilized volunteers from more than a dozen unions and made extensive phone outreach to recruit delegates. Flyers with delegate slates were distributed, and many volunteers wore "Jackson for Maine" shirts from his previous gubernatorial run. Jackson previously received Bernie Sanders' endorsement in the gubernatorial race and had campaigned with Platner before calling for Platner's withdrawal after reports of sexual assault allegations, which Platner denied and then withdrew his candidacy. Some voters reported receiving numerous calls and texts from multiple campaigns and expressed concerns that advance-slated delegate slates limited their participation. Richard Zandler, an uncommitted delegate, said his lack of affiliation with a candidate reduced his chances of being selected. Shah said his campaign would keep up pressure and continue persuading undecided delegates. Jackson posted a social media video thanking supporters and indicating that the campaign was moving forward.