GOP Sen. Susan Collins has won re-election against Democrat Sara Gideon (D) in Maine, AP projects.

Why it matters: Collins has served in the Senate for 24 years, but has faced tougher re-election battles in recent years. Democrats had expected a defeat for the moderate Republican to play a crucial role in their efforts win back the Senate.

Context: Collins had faced backlash at home for frequently voting with President Trump, who is unpopular in Maine. The senator came under fire for backing Supreme Court Justice Brett Kavanaugh and voting to acquit Trump during his impeachment trial.

• Gideon, who serves as Maine's House speaker, latched on to Collins' drop in popularity, launching a moderate Democratic bid that has promised pragmatic policymaking.
• Cook Political Report had rated the Maine Senate race as a "toss up."