DelBene's victory allows her to now transition to the November election, where she will face Republican John Koster, who easily won the top-two primary race as the only GOP candidate in the 1st District field. The four other candidates divided the remainder of the votes, with political activist Darcy Burner coming in third during returns Tuesday night.
The district had such a rancorous battle among its Democrats in recent weeks that U.S. Sen. Patty Murray, a fellow Democrat, stepped in to implore the candidates for calm. The seat is already ranked as one of the most expensive races in the country.
Washington's 1st Congressional District was carved during the redistricting process to be a tossup for Republicans and Democrats. It stretches from eastern King County areas such as the wealthy enclave of Medina all the way to the northern border.
DelBene, who lost in a 2010 bid for Congress, has spent some $2.3 million in the race while putting about as much of her own money into her campaign. That far outpaced fellow Democrats such as Burner and former state Rep. Laura Ruderman, who each spent about $470,000.
Combined, the candidates have already spent close to $4 million, making it the seventh most expensive U.S. House race in the country, according to numbers compiled by the Center for Responsive Politics.
DelBene and Koster both said the race would be centered around jobs and the economy - though they offered different visions on the topic, following the leads of their national party leaders.
DelBene said she would focus on aiding the middle class, working in part to let tax cuts expire for high-wage earners. She said her message was resonating with voters this year, despite her loss two years ago.
"This is a very different climate," DelBene said in an interview. "People are frustrated with Congress. People want to know who is going to stand up and get results for us in Congress."
Koster said he would emphasize smaller government and private-sector job creation, including the renewal of tax cuts for the wealthy.
"If you don't do that, you really cripple this economy, which is struggling as it is," Koster said. He expects DelBene to keep putting money into the race, but he thinks his message will win even if he's working with a smaller campaign budget.
The 1st District is an open seat this year due to the departure of Jay Inslee, a Democrat now running for governor. That will complicate things for some voters, as the election also will decide who will finish Inslee's term representing the old 1st District boundaries, which cover Seattle's northern suburbs.
Candidates, not wanting a disadvantage in the full-term race, have entered both contests - leaving some people to see the same person twice on the ballot. Koster and DelBene also won the primary race for that short-term seat Tuesday.
All of the state's 10 congressional districts are on the ballot. But besides the 1st District, only two U.S. House races have no incumbent.
In the 10th District, a new seat anchored around Olympia, TVW founder Denny Heck won comfortably on Tuesday as he picked up the establishment Democratic support over fellow party member Jennifer Ferguson. Pierce County councilman Dick Muri advanced after finishing second in the top-two race, besting fellow Republican Stan Flemming - also a county councilman - after a heated primary contest between the two.
Washington got the 10th District in this year's redistricting process because of a decade of population growth.
Bill Driscoll, a Marine Corps veteran who served combat tours in Iraq and Afghanistan, ousted several other Republicans to advance in the 6th District race. He will face state Sen. Derek Kilmer, who handily won the primary as the only Democrat in the race.
Driscoll is a Weyerhaeuser descendent who was able to put in about $500,000 of his own money to help buoy his campaign, bringing his total to $870,000. He still trails in fundraising against Kilmer, who has raised some $900,000.
The 6th District, which has an open seat because of departing Democratic Rep. Norm Dicks, covers much of the Olympic Peninsula.