I'll hold off on discussing what did in former GOP candidate Scott Walker. Instead, I'll focus on the important news about the nomination process: Winnowing works. Even in the era of super PACs. The harsh logic of the nomination process pushes losing candidates out of the race, until only a winner remains.

Remember: We began this last year with about 22 politicians doing the things that politicians do if they're running for president.

Five of those — Mitt Romney, Mike Pence, Rob Portman, Bob Ehrlich and John Bolton — dropped out before formally announcing their candidacies.

Now two more — Walker and Rick Perry — made it to the announcement and debate stage, but not as far as the Iowa caucuses.

Of the remaining 15, only seven — Jeb Bush, Marco Rubio, John Kasich, Mike Huckabee, Bobby Jindal, Chris Christie and Rick Santorum — have conventional qualifications for the job and are within the party mainstream on questions of public policy. The chances are very good that one of them will wind up on the stage in Cleveland next year as the nominee. And of those seven, only Bush, Rubio and maybe Kasich are showing any forward momentum at all. Nonetheless, candidates have rebounded from seeming done to becoming a serious challenge for the nomination.

As David Karol pointed out at the Monkey Cage, this has been the pattern in recent Republican contests: The losers drop out early. It's not entirely clear why this happens more on the Republican side than among Democrats (for example, Martin O'Malley is chugging along with no apparent support). In my view, it's very possible that something is happening below the surface; Republican Party actors are coordinating to gently (or maybe not so gently) apply pressure on losing candidates to exit at this point.

I'll also note that serious candidates are probably more likely to drop out than those such as Ben Carson or Donald Trump: Tim Pawlenty exited early in 2012; Walker and Perry are out for 2016, but Michele Bachmann and Newt Gingrich hung on longer in the 2012 cycle, and so might several implausible nominees this time around.

The bottom line is that we may actually be getting very close to having a nominee. It's probably down to just two or three candidates. And it wouldn't be surprising if one or more of the plausible nominees joined Walker and Perry on the bench before Christmas.