By Byron York

NEA Columnist

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, somewhat improbably, 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 the 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 recently. Teachers' unions have also been hit hard.

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, AFSCME, the AFL-CIO and others will spend tens of millions, perhaps more than $100 million, in an effort 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."

Wasserman Schultz later apologized, but she had gotten her message out.

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. The first probe did not touch Walker, and a second investigation -- into allegations he illegally coordinated with conservative groups during the recall fight -- has been stopped at least temporarily by a federal judge.

The prosecutions seem to fit the definition of "lawfare" -- that is, the use of legal action to pursue political goals. Veteran legal writer Stuart Taylor recently reported the prosecutor in the case may have been motivated by appeals from his wife, a school union shop steward who had been "repeatedly moved to tears by Walker's anti-union policies."

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 reason is that Walker may well be a contender for the 2016 Republican presidential nomination -- but only if he wins re-election first.

Walker is already admired in GOP circles for his stand against the unions. A victory could further strengthen his appeal in early-voting Iowa. "If Scott Walker, for a third time, puts up a GOP victory in a blue state, it will be a selling point that would be difficult for other candidates to match," says David Kochel, a veteran Iowa Republican strategist.

While not exactly a secret, the race hasn't gotten the attention of some of the big Senate showdowns. "The race may be flying under the radar because folks assumed Walker would be re-elected, given his success in the recall," says Charlie Sykes, a local radio host. But Sykes reminds us that President Obama won Wisconsin twice, and the governor still faces "the persistence of Walker Derangement Syndrome and the commitment of national unions to take him out."

Polls show the race essentially tied. A recent Marquette University survey showed Walker ahead by three percentage points among likely voters, which is within the poll's margin of error. 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.