Here's how the modern Republican presidential primary race plays out: Conservatives (social and tea party) eventually unite behind a single candidate. The "establishment" unites behind some other candidate. The two face off -- with the establishment candidate winning.

Mitt Romney over Rick Santorum in 2012. John McCain over Mike Huckabee in 2008. In 2000, George W. Bush was the establishment candidate but was challenged not from the social/tea party right but from the sort-of rebel center in the form of McCain. (That center no longer exists in the party -- and hasn't since at least 2004.)

This understanding of the race is based on an underlying "lane theory" of the party. The candidates are all running for the same nomination, yes, but until the very end of the race they are effectively clumped in distinct lanes -- trying to climb over a smaller number of opponents to win that lane and make the final two.

Back in February 2015, I channeled an at-the-time unaffiliated Republican consultant's view that there were four lanes in this race. Here's how I laid it out:

Think of the Republican primary field as a series of lanes. In this race, there are four of them: Establishment, Tea Party, Social Conservative and Libertarian. The four lanes are not of equal size: Establishment is the biggest followed by Tea Party, Social Conservative and then Libertarian. (I could be convinced that Libertarian is slightly larger than Social Conservative, but it's close.)

At the time, of course, Donald Trump was just a gleam in our collective eye. The thinking was that Jeb Bush, Scott Walker and Marco Rubio would fight it out for the establishment lane, Ted Cruz would be the tea party guy, Mike Huckabee would likely be the social conservative and Rand Paul would be the libertarian.

Well, a year later, the lane theory has imploded -- and not just because of Donald Trump. Consider:

* The establishment lane is Rubio and then everyone else. Walker is out. Jeb(!) is mired in single digits everywhere. Chris Christie and John Kasich need New Hampshire surprises or they are done. And, what's more amazing than the re-ordering of the establishment lane is the fact that it might not be -- as I assumed last year at this time -- the largest lane in the field. Take any national poll of the GOP primary field. Even with Ben Carson in the middle of an epic polling collapse, the "outsider" wing of the GOP -- Trump, Cruz and Carson -- take a far larger combined percentage of the vote than does the establishment wing.

* The libertarian lane disappeared when ISIS appeared. Rand Paul's entire candidacy was premised on the idea that he could take the not-insubstantial vote for his father in the 2008 and 2012 primaries and build on that. But, the rise of ISIS -- and the increasing focus on national security as a priority for GOP primary voters -- effectively dissolved the libertarian lane, which was never all that big, before Rand ever got going.

* The social conservative lane remains deeply divided. Yes, social conservatives are helping to power Cruz in Iowa. But, in South Carolina, which has a heavy evangelical contingent as well, the thrice-married Trump is leading. Huckabee has, to date, been a nonfactor. Ditto Santorum. And Carson seemed as though he might be the social conservative darling, but his recent struggles suggest that is very unlikely to happen now.

Then, of course, there is Trump, who has exploded every "the way things always work in politics" statement during the six months (or so) he's been in the race. Trump is like one of those massive monster trucks in the "lanes" theory. Jeb, Rubio and everyone else are trying to keep their car between the lines of their lane in hopes of moving to the front eventually. Trump is steering Grave Digger all over the place -- haphazardly switching lanes and, in the process, running over all -- or at least lots -- of the other cars.

Let's try to re-assert the lane theory on the race -- based on what we know today. Rubio is quite clearly the favorite in the establishment lane, although, as Timesman Frank Bruni noted over the weekend, why isn't the Florida senator doing better in polls if he's the establishment guy? Cruz is still the tea party guy. But he is also the front-runner to be the social conservative lane winner.

All of which suggests that we should be readying for a Cruz-Rubio showdown, with Rubio the most likely winner. And that could still happen. But how to account for the Trump monster truck? It's impossible to dismiss Trump's relevance at this point in the race. But it's also impossible to fit him into the "lanes" theory. Let's say, for example, the race came down to Trump and Cruz. Cruz would, by default, be the establishment candidate in that dynamic even though the establishment of the Republican party loathes him. In a Trump vs. Rubio race, it's possible that Rubio is the choice of both the establishment and social conservative lanes -- and still doesn't beat Trump.

The lane theory just doesn't work. Or, at least, it doesn't help us understand who will eventually win any better than picking names out of a hat. Which remains my plan for predicting the identity of the eventual Republican nominee.