Republican Senator Susan Collins was challenged by Democratic nominee Sara Gideon, the speaker of the Maine House of Representatives, as well as independent candidates Lisa Savage and Max Linn.

Collins was considered one of the most vulnerable Republican senators due to her decreased polling numbers and perceived harm to her reputation, but she was re-elected by an unexpectedly large 8.6% margin, with 51% of the vote to Gideon's 42.4%.

[10][11][12][13] She said she voted to acquit because "impeachment of a president should be reserved for conduct that poses such a serious threat to our governmental institutions as to warrant the extreme step of immediate removal from office.

[17][18] Collins's campaign emphasized her efficiency as a legislator, her efforts to pass the Paycheck Protection Program and other aid for small businesses, and her growing seniority and influence in the Senate, in particular her potential chairwomanship of the Appropriations Committee.

[19] Gideon was also criticized for keeping the Maine House of Representatives adjourned for most of the year (neighboring New Hampshire had reconvened its sizably larger legislature by late spring) and for allegedly turning a blind eye to a legislative colleague accused of molesting underage girls until she was forced to acknowledge the scandal.

Max Linn, a financial planner and conservative activist from Bar Harbor, was a Trump supporter and former candidate of the Republican and Reform parties.

[25] Party primaries were initially scheduled to take place on June 9, but due to the COVID-19 pandemic and its impact on the state, Governor Janet Mills rescheduled them for July 14.

Former Google executive and political aide Ross LaJeunesse, who would have been the first openly gay man elected to the Senate, announced his candidacy in November 2019.

LaJeunesse eventually withdrew from the race in March 2020, citing the inability to continue his strategy of campaigning in person due to the COVID-19 pandemic, and endorsed Gideon.

Instead, Savage sought to appear as an independent candidate, which required 4,000 signatures, but they may be from any registered voter, not just party members, and they could have been gathered until the June deadline.

[23] Eight Independents declared their candidacy for Senate in 2020, including one affiliated with the Libertarian Party of Maine, which lost ballot access after the 2018 elections.

[181] During his presidency, Collins's stance on Trump waffled, as he did not share her moderate politics but had a strong base in Maine's sprawling, rural 2nd congressional district.