Tina Podlodowski, chairwoman of the state Democratic Party, said that even though many more votes need to be counted, the early returns Tuesday night show that "nothing is guaranteed" for incumbent Republicans as Democrats showed strength that could carry through to November.
"Democrats are within striking distance of picking up three congressional seats," she said.
In two of the districts - the 3rd and the 8th - Democratic candidates combined were taking more of the vote than the GOP candidates in early returns.
Washington is a vote-by-mail state, and voters had a deadline of 8 p.m. to have their ballot postmarked or placed in a drop box. In some of the more competitive races, results may not be known for days as most counties will update vote counts only once a day.
Just over 24 percent of the vote had been counted by Tuesday night. State GOP chairman Caleb Heimlich said that "it's still too early to come to any big conclusions."
"The eyes of the nation will be on those three seats," he said. "We will do the hard work and we will connect voters and keep those seats in Republican hands."
The contest getting the most attention is an open U.S. House seat Democrats hope to capture for the first time since the district east of Seattle was created in 1980. Incumbent Republican U.S. Rep. Dave Reichert is retiring from the 8th Congressional District after more than a decade.
Republican Dino Rossi, a former state senator who had unsuccessful runs for governor and U.S. Senate, was leading a crowded ballot and easily advanced to the general election.
Under Washington's primary system, the top two vote getters go on to November, regardless of party.
Among the dozen candidates on the ballot, one of three Democrats are expected to advance: pediatrician Kim Schrier, attorney Jason Rittereiser, or former federal public-health official Shannon Hader. Schrier took an early lead among the group Tuesday night for the second spot, followed closely by Rittereiser. In early returns, the three Democrats combined had a larger vote total than Rossi, giving Democrats hope they'll consolidate that support in November.
The other nine U.S. House seats were also contested in the primary, with the incumbents advancing to the general election.
In the 5th Congressional District in eastern Washington, Republican incumbent Cathy McMorris Rodgers advanced, along with Democrat Lisa Brown, a former chancellor of Washington State University who previously served as majority leader in the state Senate. Brown and McMorris Rodgers were nearly tied in early returns.
The expected face-off between McMorris Rodgers and Brown in November has seen a flurry of television attack ads in the Spokane market.
In the 3rd Congressional District, incumbent Republican Rep. Jaime Herrera Beutler advanced along with Democrat Carolyn Long, a professor at Washington State University Vancouver. In early returns, Herrera Beutler had a slim lead over Long. Herrera Beutler has won her last two elections with more than 60 percent of the vote.
In the one statewide race on the ballot, Democratic U.S. Sen. Maria Cantwell and Republican Susan Hutchison both advanced to the November ballot, with Cantwell taking a large share of the votes in early returns.
Cantwell, who is seeking her fourth term, easily outpaced all other candidates in the Democratic primary. Cantwell is Washington's junior senator and the ranking member of the Energy and Natural Resources Committee. Hutchison, a former GOP state party chair, was finishing second in the voting.
Susan Cahill, a 66-year-old retired state employee in Olympia, said she dropped her ballot off at the mailbox on Sunday. She said she's a Democrat who has also voted for Republicans. But she said that her top of the ballot vote went to Cantwell.
Cahill said people in her community seem more engaged this year in part because of their reactions to President Donald Trump.
"I'm real impressed with a lot of my neighbors and friends," she said. "They're more vocal. There are more yard signs."
In 76 of the 123 legislative races on the ballot, there was no contest, with 15 races unopposed. In 61 seats, there were only two candidates running, all of whom automatically advanced to the November ballot.
Seventeen of the races are for open seats with no incumbent: 14 in the House and three in the Senate. Democrats currently hold a one-seat advantage in the Senate, and a two-seat advantage in the House.
Associated Press reporter Nicholas K. Geranios contributed from Spokane, Washington.
Sign up for "Politics in Focus," a weekly newsletter showcasing the AP's best political reporting from around the country leading up to the midterm elections: http://apne.ws/3Gzcraw
Copyright 2018 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.