Voters in Massachusetts' 5th Congressional District are heading to the polls to fill Edward Markey's old seat in the U.S. House.

Tuesday's contest pits Democratic state Sen. Katherine Clark of Melrose against Republican Frank Addivinola, a Boston attorney.

They have staked out very different positions in what has been a low-profile race.

The district stretches from the coast to communities north and west of Boston including Waltham, Framingham and Medford. It is heavily Democratic.

Markey resigned the seat earlier this year after winning a special election to fill John Kerry's U.S. Senate seat. Clark and Addivinola won their parties' primaries in October.

Two other candidates are on the ballot — Wellesley resident James Aulenti and Arlington resident James Hall.

Tuesday's winner will face voters again next year.

Polls are open from 7 a.m. to 8 p.m.