Scott Walker terrifies labor - and that's why he's a serious contender for the Republican primary

"Scott Walker is a national disgrace."

With those six words, Richard Trumka, the head of the nation's largest union, the AFL-CIO, welcomed perhaps the Republicans' most promising candidate on domestic policy to the race.

Walker is a relative latecomer, the 15th candidate to declare he or she is running in the 2016 Republican primary. But Scott Walker is a candidate to take seriously.

Walker has a history of winning combative battles in his home state - which has been a blue state in presidential politics since Ronald Reagan - over ideologically sacred institutions for Democrats (unions, mostly).

In 2011, in his most famous battle, he successfully curtailed the collective-bargaining power of public-sector unions in Wisconsin with a law known as Act 10. This gave all government workers "little reason to pay dues to a union that can no longer do much for them," in the words of the New York Times' Steven Greenhouse. During the fight, 100,000 protesters and labor activists descended on Madison. But they eventually lost the battle to Walker, as it passed and was signed into law.

Walker won again in 2012, when he successfully fended off a recall election on his governorship organized after the union battle, becoming the first state governor in US history to stave off a recall. He was then re-elected again in 2014, by a handy margin.

Earlier this year, Walker made Wisconsin the nation's 25th "right to work" state, which prohibits unions that represent a certain company from forcing all workers at that company to pay dues. (Again, it's a tactic to curtail the budget of a union and therefore its collective-bargaining power.)

In the New York Times, Monica Davey writes that not only does this and his previous union-busting legislation help Walker's image nationally, it also helps the party. The weaker unions are in Wisconsin in 2016, the harder it is for them to mobilize for Democrats, which is potentially the answer to turning Wisconsin red.

Finally, Walker has also begun the process of dismantling the tenure system at the University of Wisconsin. The latest state budget slashed the budget of the university system, removed enumerated protections for tenured faculty from the books, and gave tenure decisions over to the Board of Regents for the school (16 of 18 members are appointed by the governor).

In addition to all of the incredibly popular Republican policies Walker has enacted, he happens to also embody the average Republican voter, as Jonathan Allen points out in Vox:

Walker, who is white and male, will be 49 on Election Day in 2016. He is the son of a Baptist preacher who points to God's will as the force behind his political career. He lives in Wauwatosa, a city in the suburbs west of Milwaukee that has a small-town feel, and he attended Marquette University but did not graduate.

Walker is someone the party base can easily look at and see a bit of themselves. It's why Republican voters nationally view him favorably by a 30-point margin so far - with room to improve.

All of this said, Walker has his own troubles: He's been all over the map on immigration, and he's very new to the foreign-policy spotlight. He once compared taking on protesters of his 2011 anti-union campaign to taking on ISIS. And with all of the focus on unions, he runs the risk of appearing to be a one-issue candidate, which doesn't play well on the national state. But he has a foundation, and he is worth taking seriously.