Sen. Herb Kohl (D) was largely expected to win a fifth term in 2012. But his sudden announcement not to seek re-election in Wisconsin creates a vacancy in a state where the GOP made substantial gains and knocked off former Sen. Russ Feingold (D) in 2010.

For this reason, Roll Call Politics has moved Wisconsin’s 2012 Senate contest from Leans Democratic to Tossup.

Democrats say they’re confident because of voter registration swings in recent years. And President Barack Obama’s place atop the ticket may give the eventual Democratic nominee a boost in Wisconsin — a state the president won by 14 points in 2008 — but Kohl’s decision does far more harm to a Democratic party already forced to defend incumbents and other open seats across the nation.

Republicans will compete in the Badger State and hope to ride their 2010 gains next year. But state Senate recall elections this summer sparked by the legislative budget and labor fight, and a potential recall election targeting Gov. Scott Walker (R) next year could also shift the political climate.

Look for the Wisconsin Senate contest to become among the most hotly contested races in 2012.