As Republican Party leaders split between the #NeverTrumps and those who say they will support Donald Trump — or even offer him their endorsement — the emerging conventional wisdom is that the GOP is on the verge of a significant divide. It’s an extraordinary moment; American parties don’t permanently splinter very often — they are remarkably stable, resilient institutions. But sometimes factions or leaders wander off for an election cycle, or two, or more. What do these splits look like? What are the warning signs, and what happens post-breakup? What should we expect if the GOP does fall apart?

The geographic split

If we take a long view of how American political parties form broad, diverse coalitions and how those coalitions fall apart, one thing that stands out is geography. Specifically, parties that rely on the South as part of their coalition have eventually fallen apart. History gives us two examples of this.

First, the enigmatic, extinct Whig Party, which competed in presidential elections — with a few victories — across the North-South boundary from 1836 to 1852. The Whigs offer a playbook for party disintegration. Step 1: Find a major issue that is likely to divide the country along geographic and ideological lines. Step 2: Refuse to take a party position on it. In the Whigs’ case, the issue was slavery. The Kansas-Nebraska Act of 1854, which allowed new states and territories to decide whether they would be slave or free, pitted Northern and Southern Whigs against one another. That divide, in turn, spurred the formation of the Republican Party. A group of former Northern Whigs, including Abraham Lincoln, organized politically around the idea of preventing the expansion of slavery, and the GOP was born.

The second example of a dissolution of a North-South coalition took place over the course of the 20th century with the New Deal Democratic Party. At the 1948 Democratic convention, Southern delegates walked out in response to Hubert Humphrey’s speech in favor of civil rights. Aggrieved Southerners formed their own “Dixiecrat” ticket, which was listed as the official Democratic ticket in some states. It captured 39 Electoral College votes, but Democrat Harry Truman still won the election. The tensions between Northern and Southern Democrats continued through the 1950s, and there would be one more major Southern defection — George Wallace’s 1968 candidacy — over the course of the long breakup. Pinpointing an exact date for when the split was complete is difficult. Some Southern states returned to the Democratic fold to support the candidacies of Jimmy Carter and Bill Clinton, but by 2004, the South had become reliably Republican in presidential elections.

What it means for 2016: Unlike the durable splits of the past, the Republican Party divide of 2016 (if it happens) may not take place along geographic lines. The contemporary GOP doesn’t depend on either coast or the more liberal upper Midwest — the areas that find Trump most objectionable — in presidential elections. Rather, in addition to the South, whose voters seem fine with Trump, the party’s stronghold is the interior West — the Rocky Mountains and the Plains states. The basis for this coalition is ideological similarity on economic and social issues. So watch what happens when more of these states hold primaries; the results may hold clues as to how those voters will react if Trump becomes the Republican standard-bearer.

These historical examples also illustrate the role of parties as organizations. The Whigs were an idea-based movement. But they weren’t quite sure about basic functions of political parties such as campaigning. So while the antebellum Democrats, who also encompassed warring factions on the slavery issue, had a strong party organization that helped them keep it together through the sectional crisis and the Civil War, the Whigs lacked organizational strength and fell apart.

That’s not a problem for the 2016 Republican Party. As impotent as the Republican National Committee has seemed at times during this campaign, the GOP has healthy party organizations in most states. That infrastructure could help the party weather the Trump storm.

Rogue personalities

Party splits have also occurred when a popular figure bolts the party, taking supporters with him (so far it’s always been a him — in 2016 and beyond, who knows?). The prototypical example is Theodore Roosevelt, who in 1912 sought the Republican Party’s nomination in the first-ever presidential primaries but ran on his own ticket after William Howard Taft was nominated at the convention. The movement didn’t outlive its leader, though, and the Republican coalition came back together until the Great Depression.

What it means for 2016: The Republican split between Taft and Roosevelt is probably the closest parallel to what the GOP may be experiencing now. But that’s not because of any substantive similarities between Roosevelt and Trump. Rather, the issues that preceded the break, especially those that motivated the debate at the 1912 convention, were about the practice of politics rather than the substance. The Progressive movement that Roosevelt was a part of sought to elevate rank-and-file voters and break down political institutions that tamped down the popular will (those smoke-filled backrooms, for example). A strong presidency, in particular, was presented as a way to channel public opinion into successful public policy. This year, Trump has run a sort of cartoon version of a campaign about these same issues — about the people versus elites. The fate of party factions that are organized around a particular personality highlights how organization matters — it helps to have a coalition that’s built on something more than loyalty to a single politician.

The split that wasn’t really

In the late 1970s and early 1980s, the Republican Party was in the midst of a significant transition as evangelicals became an “anchoring movement” and conservative positions on gender-related issues such as abortion became part of the platform. It looked like the party might splinter over this. President Gerald Ford held onto the 1976 nomination despite a formidable challenge from conservative movement favorite Ronald Reagan. George H.W. Bush, contending for the nomination in 1980, was connected with support for abortion rights and contraception access. After Reagan won the nomination that year, John Anderson, a moderate Illinois congressman, launched an independent candidacy, with plenty of criticisms for both parties. Bush joined the Reagan ticket and downplayed their differences on social issues, but economic conservatives who leaned pro-choice remained in the party. An enduring split between social conservatives and the older, more socially moderate wing of the party never came to fruition. Ideological sorting is responsible for some of this, with liberals simply changing parties and identifying as Democrats. And the different wings of the party settled on a series of mutually satisfactory nominees, from Reagan to George W. Bush.

What it means for 2016: Tensions between the social and economic conservative segments of the party don’t appear to be what’s driving the current rift. But perhaps they have contributed to the division between the party elites and their voting base, which appears to be more conservative on immigration but less consistently conservative than elites overall.

The emerging fight within the Republican Party doesn’t look exactly like any of the major party splits of the past. There’s not a clear issue difference at the center of it. Trump is coming in from outside the party; he’s not a former president like Theodore Roosevelt. And party organizations have changed to place more emphasis on voter input — one of Roosevelt’s chief goals — even as elites play a significant role in managing conflict among factions. That role may become even more important as we head into the later primaries and the convention.

If the GOP does split, it may be for only one or two election cycles, with Trump playing the role of Roosevelt. It’s also possible, however, that the rise of Trumpism within the Republican Party will alienate the Plains states and interior West. How those states vote during the GOP primary could provide some clues (few have voted so far, but Trump has underperformed in Kansas and Oklahoma). Or maybe if Trump wins the nomination, the #NeverTrumps will sleep on the couch for a couple of months but all will be forgiven by the time the general election rolls around.

Regardless, the mere fact that a party splitting seems so plausible is pretty amazing — it doesn’t happen often.

Check out our live coverage of today’s primary elections.