With all precincts in the Boston-area 5th District reporting, Clark, a state senator, received 65.9 percent of the vote to 31.7 percent for Addivinola, an attorney, Washington publication Politico reported.
Clark, 50, will succeed Rep. Ed Markey, another Democrat who had held the seat for 37 years until giving it up this year after being elected to the U.S. Senate.
No Republican has won a U.S. House seat in Massachusetts since 1994, Politico noted.
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