PORTLAND, Maine (AP) — Maine’s two-term Democratic Gov. Janet Mills will run for the U.S. Senate seat held by veteran Republican Sen. Susan Collins next year, according to two people familiar with Mills’ plans on Friday, setting up a potential matchup between the parties’ best-known figures in a state where Democrats see a chance to gain a seat in their uphill quest for the majority. Mills was the top choice of national Democrats to try to unseat Collins, who has held the seat since 1997, and