Scott Walker's signature Monday on legislation imposing new restrictions on labor organizing made Wisconsin the nation's 25th "right-to-work" state. But more significantly, it marked Walker's return to the union-busting politics that have defined his tenure as a blue-state governor and helped push him into the top tier of Republican presidential contenders.

In working the circuit of early primary cattle calls, Walker's pitch to GOP voters is relatively straightforward: He's a "fresh new face" (in contrast to, say, Jeb Bush), and he's actually succeeded in enacting conservative policies in a state that hasn't voted Republican for president since 1984. As Walker likes to remind audiences, he has also beat back aggressive Democratic challenges three times in a span of four years, including a 2012 recall election that served as a referendum on his push to enact legislation limiting collective bargaining for state employees. "We did it without compromising," Walker boasted at the Conservative Political Action Conference last month. "We stood up and said what we were going to do. And we did."

If you're a conservative primary voter, it's an appealing message, especially when compared to the comparatively paltry record of the Republican Congress in getting results in Washington. Yet Walker's ongoing battle with unions could risk becoming a defining issue for him, and not necessarily in a positive way. This was most clearly illustrated when the one-term governor, obviously not well-steeped in foreign policy, used the 2011 law as an awkward punchline to suggest his victory over Democrats and organized labor qualified him to take on ISIS as commander in chief. "If I can take on 100,000 protesters, " he said at CPAC, "I can do the same across the world.” The clumsy boasting demonstrated his weakness in foreign affairs, but it also came close to exposing him as a one-issue pony. (For an example of how this can become damaging in a presidential race, see: Giuliani, Rudolph W.)