(CNN) - Polls are open in Massachusetts' 5th Congressional District, where voters are casting ballots to fill a vacant seat in the U.S House of Representatives that was long held by Democrat Ed Markey.
The winner of the election between Democratic state Sen. Katherine Clark of Melrose and Republican attorney Frank Addivinola of Boston will serve the final year of Markey's term.
The longtime Democratic congressman stepped down in July after winning a special U.S. Senate election a month earlier to fill out the term of former Democratic Sen. John Kerry, who became U.S. Secretary of State at the beginning of the year.
The district, which stretches from coastal communities north of Boston to the suburban cities west of the city, including Framingham, Medford, and Waltham, is heavily Democratic, with Barack Obama winning two-thirds of the vote in the 2008 and 2012 presidential elections.
Markey was first elected to the House in 1976, and grabbed 76% of the vote in his 2012 re-election.
As she's run for Congress, Clark has highlighted equal pay for women, reducing gun violence, and increasing the minimum wage. Addivinola has run as an anti-Washington candidate and has pushed for a smaller federal government. The two candidates have widely divergent views on many issues, from abortion to the federal health care law.
Clark has greatly outpaced Addivinola when it comes to fundraising, and secured the backing of Markey and the state's other Senator, freshman Elizabeth Warren.
Two other candidates are also on the ballot.
– CNN Political Editor Paul Steinhauser contributed to this report.