The tight race for U.S. Senate would have to get noticeably tighter to trigger a mandatory recount.
Although tens of thousands of ballots have yet to be counted statewide, including more than 100,000 in King County alone, incumbent Democrat Patty Murray's current lead over Republican challenger Dino Rossi (722,396 to 708,391 as of the latest election night tabulation) is beyond the one half of 1 percent margin that would trigger a mandantory machine recount under state law.
A machine recount also can triggered in statewide races if the the margin between the candidates is less than 2,000 votes. A mandatory hand recount is conducted if the margin falls below 1,000 votes and one quarter of 1 percent of total ballots cast.
The state Elections Division has a fact sheet on recounts that can be found at this link.