The Cook Political Report tells us (subscription required):

Without Appropriations Chair and 20-term Democratic Rep. Dave Obey on the ballot, GOP Ashland County prosecutor Sean Duffy suddenly has a more realistic shot at a seat in Congress than any other reality TV contestant-turned-candidate before him (which, we know, isn’t saying much). In the current political environment, any heavily working-class seat that falls close to the national partisan average (PVI D+3) isn’t the type of open seat Congressional Democrats want to defend. President Bush came within one percent of carrying this seat in 2004.

As a result, this is no longer a “Likely Democratic Seat.” Instead, “the current enthusiasm gap between the parties and the competitiveness of this district at the national level warrant moving WI-07 from the Likely Democratic column to the Toss Up column.”

And we will soon get a preview from a formerly sure-bet Democratic district: “The most important district to watch over the next month continues to be PA-12, where the May 18th special election will tell us something about voter intensity and attitudes in blue-collar areas Democrats have represented for a long time.” It seems that the Democrats’ problems are snowballing — with each resignation, others are considering heading for the hills, more seats come into play, and more vulnerable members must be defended. This is how wave elections come about.