Susan Collins

Senator Susan Collins of Maine, a moderate Republican who has represented her state in Washington for three decades, is considered one of her party’s most vulnerable incumbents in the midterm elections.

Here are five things to know about Ms. Collins, 73, of Bangor, who is running unopposed in the Republican primary on Tuesday and is expected to face Graham Platner, a Democrat, in the general election.

1. She is the longest-serving Republican woman in Senate history. When Ms. Collins was first elected to the Senate, the Nintendo 64 was brand-new, the movie “Men in Black” was in production and Louisiana was still considered a swing state. The number of women serving in the 100-seat Senate was in the single digits. Ms. Collins, an early supporter of gay rights, rose on a middle-of-the-road message in the 1996 election. A daughter of two mayors of Caribou, Maine (her father was a World War II veteran), she spent years working in government in Washington, Maine and Massachusetts, and arrived in the Senate with the reputation of a workhorse. She has maintained that brand: Last week, she became the first senator to reach 10,000 consecutive votes cast without an absence. She is the chair of the powerful Appropriations Committee, and is known for securing millions of dollars for projects across Maine.