Of course it's important which party controls the House and Senate. But for Republicans concerned about the party's 2016 presidential prospects, one key race this November isn't for control of Capitol Hill. It is the fight for governor of Wisconsin.
Democrats have been gunning for incumbent Gov. Scott Walker since he and the Republican-controlled state legislature passed Act 10 -- a measure curbing the collective bargaining powers of some public workers -- forcing them to contribute more for their health care and pensions, and ending the automatic collection of union dues.
It's hard to remember the incredible intensity that surrounded passage of Act 10 three years ago. Democratic lawmakers fled the state rather than allow a vote on it. Protesters took over the state capitol. There was an ugly Supreme Court fight. But it became law.
In the years since, Act 10 has been very good for the state budget. The measure has saved the state somewhere between $2 billion and $3 billion, mostly in pension costs.
On the other hand, Act 10 has been very bad for public-sector unions. "We've lost 70 percent of our membership in the state," Lee Saunders, president of the American Federation of State, County and Municipal Employees, told The Washington Post.
Walker's law is the most devastating blow ever struck to union domination of public services. It is probably not an exaggeration to say that revenge-seeking organized labor will pursue Walker to the grave, and perhaps beyond. This year, unions will spend tens of millions, perhaps more than $100 million, to unseat him in favor of Democrat Mary Burke.
"We have a score to settle with Scott Walker," Saunders told the Post. "He took collective bargaining away from us. He stole our voices, in a state where we were born."
The rhetoric around the Wisconsin campaign reflects its intensity. During a visit to Milwaukee earlier this month, Democratic National Committee chairwoman Debbie Wasserman Schultz said: "Scott Walker has given women the back of his hand ... What Republican tea party extremists like Scott Walker are doing is they are grabbing us by the hair and pulling us back."
Apart from politics -- or rather, in a different arena of politics -- Walker has been the target of two investigations by state prosecutors who seem determined to bring him down.
So Walker has a lot of fights on a lot of fronts. But the biggest by far is re-election.
The race is important to the national GOP for two reasons. One is that a Walker victory would validate and solidify Act 10, while a loss would undermine it. The second is Walker may well be a contender for the 2016 Republican presidential nomination -- but only if he wins re-election first.
Polls show the race essentially tied. Republicans are right to be nervous.
Of course, despite all he has faced, Walker has prevailed before. The GOP needs him to do it again.
Byron York is chief political correspondent for The Washington Examiner.