It's hit that point in the election cycle where the competitive nature of Congressional races are shifting rapidly. Unfortunately for Democrats, the overwhelming majority of those shifts are in favor of Republicans.
As Florida has becomes more of a headache for Senate Republicans three potential pick up opportunities are looking better and better for the GOP. CQ Politics is moving the Senate races in California, Washington and Wisconsin into the Tossup category from the less competitive category of Leans Democratic. All three seats are currently held by Democratic incumbents who weren't even considered to be in any real danger at the beginning of the cycle.
Senate Landscape Shifts Farther in Favor of GOP - Yahoo! News
There's one hopful race for democrats - Florida, where Crist is currently neck and neck with Rubio. Since nobody really knows what Crist is in terms of party affiliation, it'll likely be the most watch race this fall. Can't wait for the debates.