The first Washington state U.S. Senate contest of the new millennium was one of the most dramatic ever: The battle between incumbent Republican Slade Gorton and Democratic challenger Maria Cantwell (eventually won by Cantwell) was so close days went by before its contours were clear.
Haven't been any like that since. Cantwell won a solid re-election in 2006, and the senior senator, Democrat Patty Murray, did the same in 2004 against a solid challenger, Republican Representative George Nethercutt. These were not close calls.
Murray is next up again, next year (and she's expected to run). No clear challenger has emerged. There will be one, of course; Senate seats just don't go uncontested, and it would be bad politics to give a senator a free ride. Whether she draws a challenger as strong as Nethercutt is another question. And since 2004, Washington has become more Democratic.
On the national Daily Kos site, a lot of this is reviewed today alongside some new poll numbers.
These check her favorable/unfavorable numbers (55%/40%, not great but suggesting no re-elect problems). They also pit her against two of the better-known and probably stronger Republicans in the state, Attorney General Rob McKenna and 8th District Representative Dave Reichert; they show her prevailing 55%-39% and 53%-40% respectively. (Her name ID is also much higher than theirs, so that may give her some extra help in this polling.) Neither McKenna nor Reichert are likely to run against her, though, and almost any other Republican is unlikely to do as well.
If she winds up with a little-known opponent, all this may suggest why.