Three Democrats are in a close battle for second place in the all-party primary for Washington’s 1st district, a poll released today found, with ballots hitting voters’ mailboxes by the end of this week.

Snohomish County Councilman John Koster, the lone Republican, took 36 percent in the poll conducted by Seattle-based consulting firm Strategies 360.

A line of Democrats finished behind him, with former Microsoft manager Darcy Burner taking 12 percent, former Microsoft executive Suzan DelBene 11 percent, state Sen. Steve Hobbs 7 percent and former state Rep. Laura Ruderman 3 percent. Two others finished with 2 percent or less, with 26 percent undecided.

The top two finishers in the mail-only Aug. 7 election will face off in the general. The 1st district is the most competitive in the state. Former Rep. Jay Inslee (D) vacated the seat in March to focus on his bid for governor.

The poll of 500 likely voters was conducted July 15-17 with a 4.4-point margin of error.