The Cook Report (subscription required) explains:
Republicans scored a late but important recruiting success yesterday when businessman and former state Sen. Dino Rossi announced that he would challenge Democratic incumbent Sen. Patty Murray. Rossi’s announcement puts another Democratic-held seat in play. The race has been in the Solid Democratic column, but is moving to Toss Up, bringing the total number of competitive Democratic seats to 11.
As more seats come into play, the problems for the Democrats multiply. Spend money on Connecticut or Washington? Forget North Dakota and Delaware — they’re gone. How much money does Barbara Boxer need? And so it will go. Eleven seats doesn’t by any means guarantee or even make probable 11 GOP gains. It does, however, greatly increase the chances of 7-8 seats. And that’s more than enough to filibuster virtually any additions to the Obama spend-a-thon — and maybe to prevent funding of ObamaCare as well.