Sen. Herb Kohl (D-Wis.) announced on Friday that he will retire instead of seeking reelection next year. As if it weren't already likely, Kohl's announcement makes it even more probable that Wisconsin will be crucial in the 2012 election.

When I traversed Wisconsin in 2008, local experts continually assured me that it was a real battleground. Presidential candidates who win the state tend to do so by tiny margins. John Kerry in 2004 and Al Gore in 2000 both carried it by less than a point. Though there are many old-school, Midwestern progressives there, Wisconsinites tend to be a bit more skeptical of the tradition than their neighbors in Minnesota. And they launched ex-Bushie Tommy Thompson's political career when they elected him governor. Why were Michigan and Iowa getting all the attention?

Because presidential candidates who win in Wisconsin also tend to be Democrats. The last Republican presidential candidate to take the state was Ronald Reagan in 1984. Particularly amid the Democratic wave of 2008, there was little doubt Barack Obama would prevail. He won by 14 points.

But the last time I spent a few minutes with some Milwaukee conservatives — there are a few — they were raring for the coming election with enthusiasm that I hadn't detected in 2008. Wisconsin, they said, might be really competitive this year.

At the least, there's going to be a lot of money spent there. The 2010 election was really bad for Democrats; the uber-liberal Russ Feingold lost his Senate seat, and Gov. Scott Walker (R) and GOP state legislators took over Madison. Walker's push to rescind the collective bargaining rights of public employees grabbed national attention, energizing both sides and inspiring campaigns to recall state politicians. That alone guaranteed more focus on Wisconsin next year, regardless of what the vote totals ultimately look like next November. The stakes are high: Without the state, President Obama's electoral map is in serious trouble.

And, now, there's an open Senate seat, with the Democrats' precarious majority on the line. Kohl is the sixth Democrat to announce that he won't contest his seat in 2012. That makes an already tough election even tougher. Democrats must defend 23 Senate seats, and it doesn't help to have popular incumbents such as Kohl stepping aside. Republicans are sure to put even more effort into Wisconsin now.

Liberals will argue that Walker's overreach on collective bargaining will swing the pendulum there back toward Democrats. And they could be right. Or the national attention and money that flow in will warp the race in ways that early prognosticators don’t anticipate. Regardless, I'll be paying attention to Wisconsin’s polls when the general election revs up.