This is a Republican poll, so take with appropriate grain of salt. But regardless the actual numbers, it's clear Rossi is the only person who can make Cantwell sweat her reelection.
Strategic Vision (R). 6/7-9. MoE 3%. (5/15-17 results)
Cantwell (D) 40 (41)
Rossi (R) 52 (50)
Cantwell (D) 50 (50)
Dunn (R) 39 (40)
Cantwell (D) 51 (49)
Nethercutt (R) 38 (38)
Cantwell (D) 55 (54)
Vance (R) 35 (35)
Cantwell (D) 50 (50)
White (R) 38 (37)
Cantwell (D) 50 (49)
McGavick (R) 36 (36)
Rossi has been very clear -- he doesn't want to move his family to DC, hence he
won't enter the race. He's clearly itching for a rematch of the governor's race, and will likely bide his time.
The rest of the potential challengers have low name ID, so what we're seeing are Cantwell's base numbers -- 50 percent. She's not out of the woods, but she's not a top tier race against any of those non-Rossi challengers. This would be at best a second-tier race at this point.