The two talked about the economy, jobs, immigration and education at a 90-minute forum at Burlington City Hall.
It was the first time the candidates faced off since the primary in their the hotly contested 1st Congressional District race.
Koster, a Snohomish County Councilman and former state legislator, said he's worked in a bipartisan fashion in both jobs and will do the same in the nation's capitol.
"We need to elect people who will get things done, who can get things done," said Koster, who is making his third bid for Congress.
DelBene, a former Microsoft vice president and state Department of Revenue director, contended her experience in the public and private sectors has prepared her better for the challenges.
"You've got to know how the real world works," she said. "We need to elect people who are focused on getting results, not on rhetoric."
Monday marked the first time Koster and DelBene sat together since the August primary. They shared the stage with the two candidates vying in the 2nd Congressional District, U.S. Rep. Rick Larsen, D-Wash., and Republican Dan Matthews.
The forum hosted by the Skagit Business Alliance drew a crowd of about 60 people.
Koster and DelBene are dueling in the 1st Congressional District, which runs from the King County suburbs of Redmond and Kirkland north to the Canadian border. It encompasses communities and farms of east Snohomish, Skagit and Whatcom counties, including the Snohomish County cities of Lake Stevens, Snohomish, Sultan, Monroe and Granite Falls.
Koster, the only Republican to enter the race, easily won the primary, with DelBene finishing second ahead of five other Democrats and an independent.
Koster, a 60-year-old former dairy operator, is campaigning as the candidate who better understands how to unclog the engine of the economy. He said eliminating duplicative regulations and revamping how Social Security and Medicare operate in the future will lay a foundation for sustained job growth.
Excess regulation and a lack of access to capital are impeding businesses from investing and growing, Koster said. And the nation's tax code needs to be overhauled, including lowering tax rates of corporations and closing of loopholes, he said.
"We have to stop the demagoguing on this issue," he said. "We have to stop the class warfare that's gone on in the past," he said.
DelBene, 50, said as revenue director she learned that businesses want clear and certain rules under which to operate.
"We need to simplify … and it needs to be done by people who understand the process," she said
She said the path to a reborn economy requires investing in education and infrastructure and ensuring the solvency of Social Security and Medicare. She also called for stabilizing the federal budget in part by allowing the tax break for high-wage earners to expire at the end of the year -- an idea Koster opposes.
Another issue raised Monday was the proposed coal terminal at Cherry Point north of Bellingham.
DelBene heralded the potential of the project to create hundreds of new high-paying jobs but said no decision should be made until the possible negative effects of more coal-laden trains passing through Snohomish, Skagit and Whatcom counties are well-studied.
"We need to get all the data on the table so we have a smart conversation on this," she said.
Koster echoed the need for a thorough analysis "so everyone has their say" but struck a more supportive stance of moving ahead.
"If we don't build it, Canadians are and the trains will come," he said.
On immigration, DelBene endorsed the so-called DREAM Act and establishing a path to citizenship for undocumented immigrants brought to the country as children by their parents.
Koster said those here illegally need to return to their home country and then apply for citizenship. He said the U.S. is a nation of laws and those laws must be respected.