It’s a reflection of Gov. Tom Corbett’s ongoing problem with bad polling numbers as well as the recent emergence of several serious potential challengers – most notably U.S. Rep. Allyson Schwartz.
Update: While Cook didn’t publish a detailed explanation of its rating change, Senior Editor Jennifer Duffy told this to PoliticsPA:
“I considered two factors: the quality of the opposition lining up to
challenge Corbett, and the consistency of the polling data (not just
on ballot tests, but job performance, approval and re-elect).”
Joining Corbett on the danger list is Florida Gov. Rick Scott. Virginia’s governorship, which goes to voters in 2013, is the other GOP-held seat in the toss up column. The Democratic incumbent in Arkansas and independent Lincoln Chafee in Rhode Island are also listed as toss ups.