Will the New Year’s Tumult Trump the Old? The biggest question for 2016 is whether the Republican Party splits apart.

In 2016—soon, in just about a month—we’ll find out if Donald Trump voters vote. They say they’re voters and not just rally-goers. In Iowa on Feb. 1 and New Hampshire on Feb. 9, we’ll know if it’s a movement or a moment.

We are going to learn a lot pretty quickly.

In the next few months we’ll find out who emerges as the not-Trump, or not-Trumps. We’ll see a battle. If it is not resolved we’ll have a clearer sense of whether this thing is going to go to the floor of the convention in Cleveland in July. In Washington, what we sloppily but handily call the GOP establishment refers to this possibility as a “brokered convention.” They do this because they think they’ll be the brokers. Will they? Or will they be combatants? If it gets to the floor the correct term will be “open convention,” in the Katie-bar-the-door sense of wild and woolly.

TRUMP16If at some point Mr. Trump appears to be on a sure glide path to the nomination, will the party establishment begin to bolt? If so, what will that look like? If Mr. Trump is done in and his supporters perceive it as the dark work of an underhanded establishment, will they bolt? What will that look like? Will it mean they go home, stay there and refuse to come out in November? Or will they mount a third party with Mr. Trump, having changed his mind once again, at the top? If that happened—here the unknowables and the potential for drama begin to spin out in creative and unexpected directions—could each of three parties garner enough support to produce an Electoral College deadlock? That would be resolved by the House, where Republicans will likely continue to control the majority of state delegations. Would GOP representatives go for the establishment Republican or the third-party Republican?

I have not seen a political cycle so confounding in my lifetime, and it could continue into a year of the most historic kind. If you love politics—the excitement, the unknowability, the to-and-fro—this is the year for you. If you take unhappy U.S. political trends seriously—the shallowness, the restiveness, the division of our polity—you will feel legitimate concern.

We could see a great party split in two. That, I think, is what I’m seeing among the Republicans, a slow-motion break. The question is whether it will play out over the next few cycles or turn abrupt and fiery in this one. Some in Washington speak giddily of the prospect, wondering aloud if the new party’s logo should be a lion or a gazelle. But America’s two-party system has reigned almost since its beginning, and it has kept us from much woe. It has provided stability, reliability and, yes, progress. The breaking or splintering of one of those parties would be an epochal event. Ross Perot in the 1990s was a one-off; the party soon enough healed back into one. Mr. Trump may be a one-off, but the divisions he’s revealed—on how on-the-ground and unprotected people feel about illegal immigration, on the deeper and more dangerous implications of political correctness, on a host of economic and cultural issues—will not, I suspect, be resolved so easily.

If the GOP breaks it will be bitter. The establishment thinks they are saving the party from the vandals—from Trumpian know-nothingism. But Republicans on the ground think those in the establishment were the vandals, with their open borders, donor-class interests and social liberalism.

The distance between the top of the party and the bottom has been growing for years, at least since 2008. The bonds between the two have stretched and stretched, and this year they began to snap. That’s the story of the year, that the snapping became obvious. Mr. Trump and the Trumps of the future are the result, not the cause. The establishment does not see this. They think it’s about him. It’s about them.

Finally, briefly and befitting an end-of-year column, what did I get right and wrong in 2015?

A year ago I said Mitt Romney should not run. “This is a moment in history that demands superior political gifts. . . . Mitt Romney does not have them. He never did. He’s good at life and good at business and good at faith. He is politically clunky, always was and always will be. His clunkiness is seen in the way he leaked his interest in running: to mega-millionaires and billionaires in New York. ‘Tell your friends.’ ” I still think that was right. Whatever is ailing the party now, he is not the answer.

A month later I said I didn’t see Jeb Bush as the front-runner but just another candidate, and one making “a poor impression.” Mr. Bush at that point was “spending much of his time in The Rooms—offices and conference rooms—with millionaires and billionaires.” He spoke their language, his family name provided entrée, his fundraising prowess was impressive. However: “There’s something tentative and joyless in Mr. Bush’s public presentations. He isn’t mixing it up with voters or wading into the crowd. So far he is not good at the podium. His recent foreign-policy speech was both bland and jangly, and its one memorable statement—‘I am my own man’—was the kind of thing a candidate shouldn’t have to say.

“What is most missing so far is a fierce sense of engagement, a passionate desire to lead America out of the morass, a fiery—or Churchillian—certainty that he is the man for the moment. In its place we see a softer, wanner I’m smart, accomplished, know policy, and it’s my turn.

An October column said: “Jeb just isn’t very good at this.” I said his victory was impossible for me to envision. Still is.

In July I thought Scott Walker would be “highly competitive” for the nomination. He was not. In August I said Ted Cruz would get “deadlier as the number of candidates winnows down.” That seems to be bearing out.

When Mr. Trump announced in June, I knew he’d hit a powerful nerve but did not see him lasting as long as he has. He was a product of voter anger, contempt and lowering standards: “We’re entering Weimar, baby.” In time I saw his power. In August I noted: “The traditional mediating or guiding institutions within the Republican universe—its establishment, respected voices in conservative media, sober-minded state party officials—have little to no impact on Mr. Trump’s rise. Some say voices of authority should stand up to oppose him, which will lower his standing. But Republican powers don’t have that kind of juice anymore.” Traveling around the country, “my biggest sense is that political professionals are going to have to rethink ‘the base,’ reimagine it when they see it in their minds. . . . America is so in play,” and the base is “becoming a big, broad jumble that few understand.”

I asserted his appeal was not limited to Republicans. My highly scientific reason is that in talking to Trump supporters it often emerged that they were Democrats or independents.

Happy New Year; let’s get through it together.