For more than four decades, South Carolina has been the decisive contest in the Republican presidential primariesthe state most likely to anoint the GOPs eventual nominee. On Saturday, South Carolina seems poised to play that role again.

Since the state moved to its prominent early position on the GOP presidential-primary calendar in 1980, the candidate who has won there has captured the nomination in every contested race except one. Given Donald Trumps overall lead in the GOP race, a victory for him in South Carolina over Nikki Haley, the states former governor, would likely uphold that streak.

We all underestimate how deeply ingrained the Trump message is in the rank and file of our party, Warren Tompkins, a longtime South Carolinabased GOP strategist and lobbyist, told me. Take the personality out of it: What he stands for, what he says hell do, and what he did as president; hes on the money.

This year, though, there may be a twist in South Carolinas usual role of confirming the eventual GOP winner: Even as the state demonstrates Trumps strength in the primary, it may also spotlight his potential difficulties as a general-election nominee. Like the first contests in Iowa and New Hampshire, South Carolina may show that though most Republican voters are ready to renominate Trump, a substantial minority of the GOP coalition has grown disaffected from him. And in a general-election rematch, that could provide a crucial opening for President Joe Biden, despite all of his vulnerabilities, to attract some ordinarily Republican-leaning voters.

Trump is essentially the incumbent leader of the party who is not able to get higher than, say, 65 percent in the primaries, Alex Stroman, a former executive director of the South Carolina Republican Party, told me.

Local observers say Haley has run a textbook South Carolina campaign, barnstorming the state in a bus, appearing relentlessly on national television, spending heavily on television advertising, and notably intensifying her criticism of Trump as unhinged and diminished. Trump, meanwhile, has breezed through the state as quickly as a snowbird motoring down I-95 from New York to Florida for the winter. Yet he has retained an imposing lead reaching as high as two to one over Haley in the polls.

I think you can argue Haley is running a fantastic campaign in South Carolina, Jordan Ragusa, a political scientist at the College of Charleston and a co-author of a history of the South Carolina primary, told me. But the pool of available voters is just so small that no matter what she does, its going to be hard for her to move the needle.

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Over the past generation, South Carolina has had an extraordinary impact in shaping the outcome of GOP presidential-nomination contests. The state moved near the front of the GOP primary calendar in 1980, when Republicans were just establishing themselves as a competitive force in the state. GOP leaders created the primary, with its unusual scheduling on a Saturday, as a way to generate more attention for the party, which had previously selected its delegates at a convention attended by party insiders.

The other key factor in creating the primary was support from Ronald Reagans presidential campaign, including Lee Atwater, a prominent GOP strategist then based in South Carolina. South Carolina did what Atwater hoped when Reagan won it in a rout, after unexpectedly losing the Iowa caucus to George H. W. Bush.

Reagans victory in South Carolina placed him back on the path for the GOP nomination and cut a mold that has endured, with only one exception, in every contested GOP presidential-primary race through 2016. Each of those races followed the same formula: One candidate won the Iowa caucus, a second candidate won the New Hampshire primary, and then one of those two won South Carolina and eventually captured the nomination. (The exception came in 2012, when a backlash to a debate question about his marriage propelled Newt Gingrich to a decisive South Carolina win over Mitt Romney, who recovered to claim the nomination.)

In 2016, Trumps narrow victory in South Carolina effectively cemented the nomination for him after he had lost Iowa to Senator Ted Cruz of Texas and then recovered to win in New Hampshire. A victory for Trump on Saturday would allow him to equal a feat achieved only by incumbent GOP presidents: sweeping Iowa, New Hampshire, and South Carolina.

Three factors, above all, explain South Carolinas enduring influence in the GOP race. One is that it reflects the overall Republican coalition better than either of the two states that precede it. In Iowa, the Republican electorate leans heavily toward evangelical Christians who prioritize social issues; in New Hampshire, where there are few evangelicals, economic conservatives focused on taxes and spending, as well as a sizable group of libertarian voters, have dominated. South Carolina is the synthesis of both: It has a large evangelical population and a substantial cohort of suburban, business-oriented Republicans outside its three principal population centers of Greenville, Columbia, and Charleston.

In a lot of ways, the state party here is a microcosm of the national party, Jim Guth, a longtime political scientist at Furman University, in Greenville, told me. We replicate the profile of the national party maybe better than New Hampshire [or] Iowa.

It has been possible for candidates over the years to win Iowa or New Hampshire primarily by mobilizing just one group, such as social conservatives in Iowa and moderate independents in New Hampshire. But because the South Carolina GOP contains so many different power centers, you have to have a broader appeal, Tompkins, who has worked in every GOP presidential primary since Reagan, told me.

The second key factor in South Carolinas importance has been its placement on the GOP calendar. From the outset, in 1980, the primary was designed by its sponsors as a First in the South contest that they hoped would signal to voters across the region which candidate had emerged as the favorite. As more southern states over the years concentrated their primaries on Super Tuesday, in early March, that multiplied the domino effect of winning the state.

Given the demographic alignment between South Carolina and a lot of the southern Super Tuesday states, and the momentum effect, it really made South Carolina pivotal, Ragusa said.

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The third dynamic underpinning South Carolinas influence has been its role as a fire wall against insurgent candidates such as John McCain in 2000 and Patrick J. Buchanan in the 1990s. South Carolinas Republican leadership has usually coalesced predominantly behind the candidate with the most support from the national party establishment and then helped power them to victory in the state. That model wavered in 2012, when Gingrich won his upset victory, and even in 2016, when Trump won despite clear splits in the national GOP establishment about his candidacy. But most often, South Carolina has been an empire-strikes-back place where the establishment-backed front-runner in the race snuffs out the last flickers of viable opposition.

All of these historic factors appear virtually certain to benefit Trump this year. Super Tuesday no longer revolves as much around southern states. But it remains a huge landscape: 15 states and American Samoa will all pick a combined 874 Republican delegates on March 5, nearly three-fourths of the total required to win the nomination.

In the limited polling across the Super Tuesday states, Trump now leads, usually commandingly, in all of them. Haley has already announced campaign appearances in Super Tuesday states through next week. But with all of the Super Tuesday states voting just 10 days after South Carolina, it will be virtually impossible for Haley to close the gap in so many places at once without winning her home state or at least significantly exceeding expectations. Like earlier underdogs, she faces a stark equation To change the race anywhere on Super Tuesday, she must change it everywhere through her showing in South Carolina.

Saturdays result could also reconfirm South Carolinas other key historic roles. Trump is now the candidate of most of the GOP establishmenta dynamic reflected in his endorsement by virtually all of the leading Republicans in Haleys home state. Hes also become the contender with the broadest appeal inside the Republican Party. Because Trump is so polarizing for the general public, its difficult to see him in that light. But South Carolina is likely to buttress the indications from Iowa and New Hampshire that Trump, as a quasi-incumbent, now has a broader reach across the Republican Party than Haley does, or, for that matter, than he himself did in 2016. In most South Carolina polls, Trump is now leading her with every major demographic group, except among the independents who plan to participate in the primary.

Yet South Carolina, like Iowa and New Hampshire before it, will also provide important clues about the extent of the remaining resistance to Trump within the Republican coalition.

Haley is likely to perform best among well-educated voters around the population centers of Columbia and Charleston. Haley must run up the score with traditional Reagan Republicans who want to actually nominate a candidate who can win in the general election, Stroman told me. She is going to be absolutely swamped in the MAGA-rich right-wing upstate, and in rural areas across the stateso she needs the suburbs and cities to turn out to hopefully keep her closer than expected.

In New Hampshire, Haley finished closer to Trump than most polls projected, because a large number of independent voters, and even a slice of Democrats, turned out to support her. Shell need a similar dynamic to finish credibly in South Carolina, where she has said her goal is to exceed her 43 percent of the vote in New Hampshire. The better the showing for Haley among independents, and among college-educated voters in the suburbs, the stronger the general-election warning signs for Trump.

Democratic voters could be a wild card on Saturday after relatively few of them turned out for the partys own primary earlier this month. South Carolina does not have party registration, which means that any voter who did not participate in the Democratic primary can vote in the Republican contest. A group called Primary Pivot has launched a campaign to encourage Democrats and independents to swarm the GOP primary to weaken Trump. If Haley exceeds expectations in South Carolina, it will be because, as in New Hampshire, more independents and Democrats turn out for her than pollsters anticipated.

Besting Trump for the nomination may no longer be a realistic goal for Haley if she loses her home state. But, after mostly dodging confrontation with Trump for months, she is now delivering a more cogent and caustic argument against him, and showing a determination to force Republicans to wrestle with the general-election risks they are accepting by renominating him. The biggest question in South Carolina may not be whether Haley can beat Trump, but whether the state provides her more evidence, even in defeat, to make that case.