So it's settled, then. Fox News will limit its Aug. 6 debate to the 10 Republicans who are polling the highest in the last five national polls. A simple, clear delimiter. Done and done.


If you look at the field right now, the top 10 goes like this:

That's Jeb Bush, Marco Rubio, Scott Walker, Rand Paul, Ted Cruz, Mike Huckabee, Ben Carson, Chris Christie, Donald Trump and Rick Perry, in that order. Just out of the running? John Kasich, Rick Santorum, etc.

Now, if you're John Kasich's people, you have any number of arguments you can make. After all, you're only 0.2 percent below Perry in the average, and that's in a bunch of polls with margins of error north of 3 percent. In fact, you've done the same as Perry in every poll except the Quinnipiac one, where Perry got 3 percent to your 2 percent. That's the entire difference.

If Perry and Kasich had tied, by the way, Fox says it would include both. But what if, say, three candidates tied? Or four? Give Santorum an extra point in a poll or two, and he's in the mix, for example. Will a candidate accept being excluded due to rounding?

What's more, the average above includes Public Policy Polling, a Democratically affiliated firm. Should it be included? Should it not? (For what it's worth, the network's guidelines are that it will include polls "by major, nationally recognized organizations that use standard methodological techniques recognized by Fox News.") For what it's worth, if you take PPP out, it doesn't change much.

But should we include Donald Trump? His numbers come from appearances in just two polls, both from Fox News. Does doing well in two polls and not existing in three others count? By the time August rolls around, the field may be settled enough that this won't be as big a deal -- but it could be! With a field this big, pollsters spend a lot more money having their callers ask people to pick from a list of 20-odd names. But if you start leaving people out, that decides when and if they appear in a debate. And that's a big deal.

Another way of looking at it: Here's what the field looks like if you use the last five polls from a month ago today.

Santorum makes it into the top 10 -- because Trump, who didn't appear in any polls, drops out. That's a big change, using several of the same polls. Notice, too, that Rubio and Cruz, who were in the debate with a lot of clearance in the first poll, almost fall out of the bottom here. As the debates approach and polling increases, there's going to be a lot more variability at the lower end of that graph. What if Fox News invites 10 people to the debate, but a poll released that morning kicks one of the participants off the list? Will Fox rescind its invitation?

None of this is Fox's fault, of course. Every four years, debate hosts struggle with whom to include and exclude. What's (partly) different this time is that the "outside candidates" are real candidates: Lindsey Graham, the respected senior senator from South Carolina, doesn't get to appear on stage in either of these situations. You have to draw a line somewhere, and Fox penciled in that line.

In short order, expect the candidates to start busting out erasers.

Update: The second debate, hosted by CNN, will round up the second tier candidates into their own debate. Which solves a lot of problems.