In early returns Tuesday night, DelBene led by 54 percent to 46 percent for Koster in the race for the open 1st Congressional District seat.
Spanning from the suburbs of King County to the Canadian border in Whatcom County, the sprawling, redrawn district was seen as a toss-up throughout the election.
DelBene, a former Microsoft executive, defeated a group of Democrats in the primary to face Koster, who faced little opposition from his own party. DelBene used more than $2 million of her own money in the campaign.
After the 2010 Census, Washington gained a congressional seat. The state now has 10.