Koster, a Snohomish County councilman and the only GOP candidate in the race, enjoyed a commanding lead in the seven-person field with 43.7 percent or 33,100 votes, in results compiled by the Secretary of State.
"I'm pleased with the numbers. I think that's where we needed to be," Koster said at his election night party in Everett. "We'll take a deep breath tomorrow and we'll go forward with our game plan."
DelBene, who spent $2.3 million of her own money in the race, is making plans to focus on November after Democrat Darcy Burner, who is in third, conceded. DelBene collected 23.3 percent, or 17,663 votes, followed by Burner with 14.6 percent or 11,051 votes.
"Darcy Burner gave me a call and congratulated me on my success," DelBene said. "We're moving forward for the general election."
Rounding out the field were Democrats Laura Ruderman with 7.1 percent, Steve Hobbs with 6.8 percent and Darshan Rauniyar with 2.5 percent and Larry Ishmael, an independent, with 2 percent.
Ballot counting continues Wednesday with the latest results getting released in the afternoon.
Koster and DelBene will compete for a two-year term in a district considered the most evenly split between Democrats and Republicans in the state, if not the nation.
The winner will succeed Democrat Jay Inslee, who decided to run for governor rather than seek re-election to Congress.
The candidate who wins in November will represent a district which stretches from Redmond and Kirkland in King County north to the Canadian border. It passes through mostly new territory of Snohomish, Skagit and Whatcom counties marked by small towns and farmland and Republicans.
In Snohomish County, it includes the cities of Granite Falls, Lake Stevens, Mill Creek, Bothell, Monroe, Snohomish and Sultan.
Meanwhile, Koster and DelBene will also duel in the special election to serve one month in Congress to finish Inslee's term.
In that race with 11 candidates, Koster received 34.8 percent and DelBene 25.1 percent in initial returns. Burner is in third with 15.2 percent. Democratic Snohomish County Councilman Brian Sullivan is fifth with 3.4 percent behind Ruderman with 9.6 percent.
Tuesday's results didn't surprise Koster, long the favorite in the regular election because he was the only Republican candidate. Polling throughout the election showed the 60-year-old Arlington area resident racking up a double-digit advantage on the others.
That's kept everybody's attention, including his, trained on the Democrats to see which of them would advance.
Burner, 41, of Carnation, led in the polls for months but DelBene, 50, of Medina, drew even around the time ballots went out and her spending on television ads and mailers kicked into high gear.
"If you spend enough, that's the results you get," Koster said, adding he expects her to keep writing checks to her campaign. "We don't need to match them dollar for dollar to beat them."
DelBene, a former vice president at Microsoft who self-financed an unsuccessful challenge of Republican Congressman Rep. Dave Reichert in 2010, wasn't apologizing for her personal spending.
"We worked hard to get our message out to voters," she said.
She also was noncommittal whether she'd continue to invest heavily in her bid for Congress.
"That hasn't been something I've decided," she said.
Tuesday's results seem to indicate DelBene didn't suffer any damage from a series of attacks launched against her by an independent political action committee, Progress for Washington, whose chief funder is Ruderman's mother.
Nor did it appear Burner's charge that DelBene was using her personal wealth to try to buy the seat carried weight with voters.
"I think focusing on the issues and focusing on the positive message really made a difference," she said.
Burner, the most left-leaning of the Democratic candidates, could not match DelBene in fundraising. She lacked funds for an ad campaign on television but did send out magazine-like mailers to a targeted group of voters.
Ruderman, 41, of Kirkland, is a former state lawmaker who mounted a visible campaign with television ads running throughout the three weeks of voting. She also benefitted from a number of supportive mailers sent out by her mother's independent political committee.
Hobbs, a 42-year-old state senator from Lake Stevens, campaigned as a moderate Democrat willing to team with Republicans on some matters. He racked up a number of impressive individual endorsements but also had troubles raising enough money to mount an effective advertising campaign.
Rauniyar, 42, a native of Nepal, is making his first run for political office. He campaigned as an "outsider" and "non-professional" candidate who could bring a fresh perspective the others lack.
Ishmael, 59, of Kirkland, is making his third bid for Congress. Running as a Republican, he lost to Inslee in 2006 and 2008 in what's soon to become the old 1st District.
Jerry Cornfield: 360-352-8623; firstname.lastname@example.org.
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