They've also launched a few accusations at one another in press releases and television ads and during recent joint appearances.
Koster jabs DelBene for not voting in elections before her first run for Congress and tries to link her with a complaint alleging political favoritism in the state agency she guided last year.
DelBene says Koster "crossed the line" by placing photos of her home on his campaign website and decries his depiction of people receiving government assistance as lazy and slothful.
His commercials label her a multimillionaire trying to buy her way to a seat in Congress. Her ads deem his views on social issues as too extreme and she routinely cites his ties to tea party supporters.
Voters in the 1st Congressional District should expect such volleys of claims and counterclaims to continue -- even as both candidates insist it's not what they want to focus on in the final days of the election.
It's "disappointing what the campaign has come down to with the mudslinging," DelBene said Friday on KUOW radio (94.9 FM) where she and Koster appeared jointly for the final time of the campaign.
The two are vying for an open seat in Congress serving an area from the King County suburbs of Kirkland and Redmond north to the Canadian border. It runs through the towns and farms of eastern Snohomish, Skagit and Whatcom counties.
It's a close race in a district with a roughly even number of self-described Democrats and Republicans. A recent SurveyUSA poll found DelBene ahead but within the margin of error.
Koster, a former state lawmaker, is in his third term on the Snohomish County Council and making his third try for Congress. He ran in 2000 and 2010, losing both times to 2nd District Democrat Rick Larsen.
DelBene, a former executive with Microsoft Corp., served as director of the state Department of Revenue in 2011. This is her second bid for Congress; she lost to 8th District Republican Dave Reichert in 2010.
A whistleblower complaint against the department is fueling a recent barrage of criticism from Koster.
The complaint from a longtime agency employee alleges that political considerations had a hand in the agency's failure in 2011 to make a Democratic donor pay taxes on the purchase of an airplane.
Koster suggests DelBene knows something and an expedited investigation is needed to find out for certain before the election.
"It was under her watch," he said. "I don't see her standing up calling for an investigation."
DelBene's response: "This is a baseless accusation from a very desperate campaign."
State Auditor Brian Sonntag has said an investigation is under way and won't be rushed because of the election. "We're not going to let anybody's political agenda in any direction affect the work we do," he said.
Koster also hits at DelBene for not voting in nine elections before running for Congress in 2010. He said she wasn't concerned enough to participate until she decided to seek political office.
"I did miss some votes. That was a mistake of my past," DelBene said on KUOW, adding Koster "does not have a perfect voting record."
She noted he missed votes in two special elections while serving in the state Legislature. Koster blamed it on failing to obtain an absentee ballot during the session.
DelBene, in Friday's radio show and an earlier television appearance, blasted the posting of photos of her home on Koster's campaign website.
"John Koster's crossed the line," she told KING-TV's Robert Mak. "He has had people sneaking around my house taking pictures. I find it indefensible and, frankly, creepy."
Koster denied anyone had been sneaking around the house and explained that the pictures were available from online sources.
DelBene takes Koster to task for criticizing recipients of government assistance in a December 2011 speech.
In the speech, Koster said the result of policies enacted under President Barack Obama is to punish "those who dare to dream, those who dare to invest, those who dare to work hard or succeed" while seeming to "reward mediocrity, dare I say it, slothfulness, and laziness, of those who choose not to do those things."
He says later that is not right that 47 percent of Americans don't pay a federal income tax. That figure -- which Republican presidential candidate Mitt Romney injected into the national political debate -- includes people living on checks from Social Security, Medicaid and veterans programs.
In an interview this week, Koster said in his speech he was talking about welfare programs and people who take advantage of them.
DelBene said voters can listen to the comments, which are in a video posted on Koster's website, and interpret them for themselves.
"When people are looking at who they're going to elect, they want to know their values," she said. To even say 47 percent of Americans are lazy and slothful "is representative of where someone is thinking."
Periodically in the campaign, DelBene's poked Koster for co-sponsoring a resolution honoring Rush Limbaugh.
"When my opponent was serving in the state Legislature he was not focused on key issues," she said.
"To have someone say Rush Limbaugh is someone they would honor and take time of our state Legislature to honor" is not a sign of a bipartisan leader or someone truly focused on resolving issues facing the state, she said.
Koster's response: "I don't know where that comes from."
Jerry Cornfield: 360-352-8623; firstname.lastname@example.org.
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