Scott Walker dropped out of the GOP race on Monday...and it won't make a bit of difference. He just figured out that he was just a dude playing a dude disguised as another dude, and all of those "dudes" were already on stage, and well ahead of him.
As I have talked about before, the "race" at this point is about coordinating on the two top candidates. While a third viable candidate definitely keeps it interesting and in the running for a cable series, the simple reality is that being a candidate outside of the top three is the same, in the end, as being "a voter."
"Wait, Walker isn't Chuck Norris?"
It's important to remember that Walker's campaign had nothing but some (serious) pre-race media headwinds. While "known" by the cognoscenti (I think that's a Democrat term, though), Walker is, in the end, a fairly unknown governor from a fairly blue state. So while he arguably "looks like" the mirror image of Clinton or Carter, his only real card (like them) is/was that he is "an outsider, ready to shake things up."
Looking around the bizarre bazaar that is the GOP race right now, the market price for "outsider, ready to shake things up" is basically zero. Trump, Carson, and Fiorina are selling that stuff straight out of the back of the truck at this point.
Walker's demise is about more than only Walker
The demise of Walker's campaign vividly illustrates the ill humors that waft through the halls of today's GOP. Walker's rhetoric is essentially composed of anti-union & anti-tax fodder, and he has ascended to his station in the party arguably because he won repeatedly in a "blue" state. Like Ohio and Pennsylvania, Wisconsin is a sort of "demographic last refuge" for the late 20th century vision of the Republican Party: disenfranchised but alert (mostly) white (largely) male voters who wish for better (read: older, misremembered) times.
Why Walker (really) did the GOP a favor
The problem for the GOP with seeking a savior---even a "savior for now"---in candidates like Walker is that his strongest appeal squarely places both the GOP's appeal and the perception of the GOP's vision of itself in the past. Setting aside clearly niche issues such as public sector union protections, what did Walker offer to the GOP that was hard for anybody else to bring? In the end, Walker is John Kasich, but from a state with fewer electoral votes. Walker is a younger Jeb Bush, but from a state with fewer electoral votes and worse weather.
Walker should have taken a cue from Paul Ryan, who declined to run this year. I'd argue that Ryan didn't run because he is savvy and knows that this ain't the year to figure out what the GOP is or should be in the next 4 years. To the GOP's credit, and foreshadowing the next wave of partisan identity crises, the GOP field is a lot younger than the Democratic one.
Walker is rethinking things because the GOP needs to as well
These events (Walker's stepping back, Ryan's demurral, and the partisan/demographic differences) are in line with the fact that GOP is amorphously coming to recognize that the GOP needs to reimagine itself. Pushed on it, I'd argue that the (perhaps temporary) wilting of Jeb Bush's campaign is a result of the same inchoate recognition by the GOP base. Simply put, there are no new ideas out there right now. The GOP needs a message that is more than simply "repeal Obamacare."
There is something that is kind of funny, and I'll leave it here for thought. Hillary's campaign's bumps in the road over the past couple of months, combined with the Sanders/Biden dynamic of there possibly being discord on the Democratic side, might have created a vacuum of panic, as it were, in the GOP ranks. The debates, the spin/noise cycle of media and blogs...these have all created an air of importance and prominence around the GOP campaign. More relevantly, they may have created a downdraft on worries about which of the GOP candidates in this free-for-all can actually fight in the (seemingly interminably far off) general election.