Currently, the Democrats hold a 27 to 22 seat lead. But the two parties and their allies have poured millions of dollars into just a few districts, mostly located west of the Cascade Mountains.
The Republicans see an opportunity to seize the Senate for the first time in a decade. Democrats want to win enough seats to shed the influence of conservatives in their ranks, which has made their control of the upper chamber shaky at best.
All of the seats in the House of Representatives and about half of the Senate are on the ballot this year.
In King County's 5th District, which covers eastern King County territory such as Issaquah and Maple Valley, Democrats are hoping to gain the Senate seat vacated by Republican Cheryl Pflug, who continues to have a public fall out with her party after she accepted a state job offered by Gov. Chris Gregoire.
Pflug withdrew her name from the race just a few days after the candidate-filing period came to a close. That did not allow the GOP to recruit a candidate. Democrat Mark Mullet won the primary with 52 percent of the vote and has also been endorsed by Pflug.
In the Vancouver area, incumbent Republican state Sen. Don Benton is being challenged by state Rep. Tim Probst. While Benton took the primary with 890 votes, Democrats are feeling confident they can unseat the incumbent.
In the 10th District, which covers all of Camano Island and parts of Skagit and Snohomish counties, Republican state Rep. Barbara Bailey received more votes in the primary than longtime incumbent Democrat Sen. Mary Margaret Haugen.
Republicans are also mounting a challenge to Sen. Rosemary McAuliffe, who represents parts of northern King County and south Snohomish County.
McAuliffe, a lead lawmaker in education issues, race has been fueled education organizations. McAuliffe is being boosted by the teacher's union, the Washington Education Association. Her opponent, Dawn McCravey, has seen nearly $250,000 of support from the pro-charter school outfit, Stand for Children Washington.
Spending by candidates, their parties and outside groups in those four races has neared or topped a million dollars each.
Democrats are expected to keep their majority in the House. Republicans would need a net gain of 8 seats to challenge the Democrat's majority in the lower chamber.