Three Service Employees International Union local groups have endorsed Dow Constantine for King County Executive. The union is the largest in the state, with 20,000 members in King County in the health care and service industries. The union endorsement for Constantine comes largely amid fears of a possible Susan Hutchison administration. “Dow Constantine has been willing to take her on, and has the vision and the experience to be the best candidate to take her on in the general election and keep King County moving in a progressive direction,” state a press release issued this morning. Over at the PI, Joel Connelly is calling it a “key” endorsement in part based on the SEIU’s success in working on behalf of Christine Gregoire and Greg Nickels. And certainly it can’t hurt Constantine’s chances. But King County is a very different animal than the city with a Republican voting bloc and Hutchison is a very different kind of candidate. The role of union endorsements really hasn’t been tested in the countywide Executive race since the very-pro-union and perpetual endorsement-getter Ron Sims has been at the helm. This year will be a big test of whether or not the SEIU push their chosen Sims successor through to November and stave off a well-known and popular television personality.