One key takeaway from the news that Wisconsin Senator Herb Kohl is set to retire is that the national Republican Party could end up coming to regret Governor Scott Walker’s overreach in a major way: It could cost Republicans a Senate seat they might otherwise have had a better shot at winning.

Kohl’s retirement means that there may now be a competitive race for a seat that would otherwise have remained securely in Dem hands. As national Republicans are pointing out this morning, Tea Partyer Ron Johnson’s victory over Russ Feingold last year suggests that this race, too, could be up for grabs.

“As Russ Feingold discovered last year, whomever the Democrats eventually nominate, he or she will have a very tough time selling Wisconsinites on the Washington Democrats’ agenda,” the NRSC said in a statement. And as Steve Kornacki pointed out recently, Feingold’s loss showed that Wisconsin isn’t as liberal as it appears.

But the key is that a lot has happened in Wisconsin since Feingold’s loss. The months long war in the state over Scott Walker’s effort to strip public employees of their bargaining rights has galvanized the Democratic Party in Wisconsin in a major way and — if polls showing the unpopularity of Walker’s proposals are any guide — has tilted independents and moderates in the state against GOP rule. It’s true that this battle has galvanized the grassroots on both sides, but the emerging shape of the recall elections suggest the left has more momentum and energy.

This fight, of course, will continue through at least the summer, when recall elections are set to take place, and perhaps beyond, if Dems can mount a credible drive to recall Walker himself. No matter what, it seems certain that Walker and his agenda will figure in the Senate race.

Some are already envisioning an epic matchup for Kohl’s seat between Feingold and GOP Rep. Paul Ryan. If so, Wisconsin could emerge as a kind of epicenter of GOP overreach, where the Dems’ theory of the electorate — that Ryan and Walker both represent a brand of ideological extremism that is badly alienating middle of the road voters — would get its ultimate test.

At any rate, if Walker ends up being a key factor that deprives Republicans of a Senate seat they might otherwise have had a better chance at winning, it would increase the price Republicans are paying for Walker’s overreach that much more. And it would be a fitting coda to a story that just keeps on going, and going, and going.