(a) The qualified electors of the state, of any county, city, village, or town, of any congressional, legislative, judicial, town sanitary, or school district, or of any prosecutorial unit may petition for the recall of any incumbent elective official by filing a petition with the same official or agency with whom nomination papers or declarations of candidacy for the office are filed demanding the recall of the officeholder.(Excerpted by ThinkProgress)
(b) Except as provided in par. (c), a petition for recall of an officer shall be signed by electors equal to at least 25% of the vote cast for the office of governor at the last election within the same district or territory as that of the officeholder being recalled. . . .
(s) No petition for recall of an officer may be offered for filing prior to the expiration of one year after commencement of the term of office for which the officer is elected.
The Swing State Project went one step further, creating a table of all WI state senators who could be recalled if constituents collected just north of 15,000 signatures in their districts. As usual, a fairly thoughtful exercise from SSP. The table includes each vulnerable senator's win margin in 2008. SSP goes on to highlight Dan Kapanke, the state senator who challenged incumbent US House Rep Ron Kind (WI-03) and lost in 2010. SSP notes that Kapanke only won his district in 2008 by 3%.
SSP concludes that Kapanke is perhaps the lowest hanging fruit for a recall effort, but there's a crucial data source that SSP has totally ignored: how Kapanke faired in his state senate district in 2010. While Kapanke had been an incumbent in 2008, it was only his first campaign as a sitting senator, and perhaps his campaign was not fully versed on using the resources of an incumbent. He also may have become more popular during his high profile campaign against Ron Kind. It's kind of hard to guess. On the other hand, the data is readily available:
It turns out that in the 2010 congressional race, Dan Kapanke lost his own state Senate district by almost 5,000 votes.