GOP candidates on stage at the CNBC debate in Colorado. (Mark J. Terrill/AP)

In the matter of Republican presidential debates, why not just make it Donald Trump vs. Ben Carson? If the debates are about picking winners and losers ahead of the primaries and caucuses, why not the two non-politicians who have lapped the rest of the GOP field?

This notion isn’t meant seriously, of course. But by the measurements being used to select the candidates deemed worthy of participation, it could be argued that it would be justifiably entertaining to stage a debate only between Carson and Trump, at least until some of the others in the race show they’ve got stronger numbers to justify including them.

If national polls are in fact the grand arbiters of things, Trump and Carson deserve to be in a special category. As of Friday, the current poll average on HuffPost Pollster puts Trump in first, at 28 percent, with Carson second at 22 percent. The next closest candidate is Florida Sen. Marco Rubio, but he’s at just 10 percent. The poll average on Real Clear Politics has Carson and Trump tied at 25 percent (24.8 and 24.6 percent respectively, to be “exact”). Rubio is third at 11 percent.

This is a different pattern than the one seen in the 2012 Republican campaign. Four years ago, a procession of GOP candidates rose to the top of the polls, seeking to challenge front-runner Mitt Romney, only to fall away. Trump has been at the top of the race for months. Carson has been on the rise since August, leaving the rest of the field far behind.

When it comes to staging debates in the year before the primaries, there are not many easy calls. Everyone can find some reason to complain about something, and this year proves the point. Last month’s debate in Colorado, which was hosted by CNBC, produced a storm of protest from the candidates, who were offended by the tone and subject matter of the questions. That came on top of irritation among the candidates about the handling of the debates by the Republican National Committee.

This debate over debates has been heightened by the announcement of who will and will not be included in Tuesday’s GOP debate in Milwaukee, hosted by Fox Business Network in partnership with the Wall Street Journal.

Based on the criteria set by the network, Fox announced that the prime-time debate would be limited to eight candidates, rather than 10, as was the case in the first three debates. That means the two candidates who have been on the main stage in three previous debates — New Jersey Gov. Chris Christie and former Arkansas governor Mike Huckabee — will be pushed out of prime time to the undercard debate.

In addition, two candidates who have participated in the previous undercard debates, South Carolina Sen. Lindsey O. Graham and former New York governor George Pataki, will not be included at all because their poll standings, based on the polls used for selection purposes, did not meet the threshold for qualification.

The size of the Republican field has certainly complicated the staging of this year’s debates. With so many candidates, it would be unwieldy to try to include everyone on the same stage. On that there is widespread agreement. It’s what to do about it that causes problems.

If the stage cannot accommodate everyone, what is the right number for a debate and how is that determined? Are eight candidates really better than 10? Wouldn’t four be better than eight? In that case, why not Trump, Carson, Rubio and Sen. Ted Cruz of Texas — the two leaders and the two candidates who are drawing attention as their poll numbers increase?

Is it better to use national polls or those from states like Iowa and New Hampshire, where the candidates are spending the most time? The stratification of the GOP field looks somewhat different in the state polls than in the national polls, and it varies between the two states. Another question: Should the big field be split in half, randomly, for each debate night, providing a different mix and therefore less predictable exchanges among the participants?

For those sponsoring debates, there is a natural tendency to seek ways to limit participation — and not just for logistical reasons. Network sponsors are looking for the kind of on-stage chemistry that will produce political drama and fireworks, high ratings and big revenues. That argues for including fewer candidates but comes at a cost of not giving the public a look at as many as possible.

It’s easy to use polls as a proxy for the public to rationalize cutting out a few candidates with dismal numbers. But on this, polls are imperfect and the cut points arbitrary. The first contests are still almost three months away, in a cycle in which conventional wisdom has taken a beating. Who should be picking winners and losers at this point?

All available evidence suggests that the Republican race is still fluid. Many voters are still shopping. Last week, the polling firms Public Opinion Strategies and Purple Strategies conducted a focus group of 10 “Wal-Mart moms” in New Hampshire. At the end, seven of the 10 said they were considering at least four candidates. They weren’t asked, but based on interviews over the past few months, it’s not likely they all have the same four or five under consideration.

Events will continue to change the shape of the race. The leading candidates will face more scrutiny and negative attacks from their rivals. What impact that will have on their standing depends in part on the seriousness of the attacks, as well as on the effectiveness of the responses. Such developments could rearrange the polls again and put a different group of candidates on the stage in future debates.

Cumbersome as it might be and whatever impact it might have on TV ratings, it would be helpful to find a more equitable way of staging debates. At this point, the public deserves to see and hear more rather than less. In the end, the voters — not the media or the polls — will serve as the ultimate arbiters of who is and isn’t a viable candidate.