A mediocre high school athlete of non-American parentage leaves his palm-tree-lined hometown to bounce between multiple colleges, eventually earning a political science degree and attending law school.

He begins his political career in an area dominated by an urban machine. After serving eight years in a state legislature and lecturing at a local college, he decides to launch a long-shot campaign for the U.S. Senate. His intra-party opposition starts out with far more money and includes a vaunted statewide official. But he excites partisans fed up with the status quo in the primary and capitalizes on a bizarrely fractured opposition in the fall, becoming an overnight celebrity in his party.

He frequently talks about his parents’ dreams while speaking at party get-togethers, but he grows restless in his first term and, like most freshmen, compiles few notable legislative accomplishments. A few years into the job, his party retakes the Senate majority. Nevertheless, just a few months later, he launches an uphill presidential bid while still in his mid-40s.

At first, pundits dismiss his prospects because his party’s front-runner is an immediate family member of the previous president, with the ability to raise $100 million and roots in his home state. But his raw oratorical talent and a pervasive anti-dynasty sentiment help him win a drawn-out, seemingly endless primary slog.

In November, his opponent is more than two decades older and had lost a race for the White House eight years before. He capitalizes on the nation’s mood for change after a two-term president of the other party, claiming a historic victory.

This is the story of Barack Obama, but it could also be the story of Marco Rubio. The striking parallels between the two, beyond the obvious ethnic barrier-breaking nature of their candidacies, make Democratic strategists terrified to face Rubio in the fall. Yet the notion that Rubio is the “Republican Obama” also makes some GOP voters hesitant to support him.

There are a lot of complex analyses of the 2016 election floating around. My own theory is quite straightforward: If Hillary Clinton is the nominee — and she remains a heavy favorite over Bernie Sanders — her fate largely rests with Republican voters’ decisions over the next few months.

If Republicans nominate Rubio, they would have an excellent chance to beat Clinton by broadening their party’s appeal with moderates, millennials and Latinos. The GOP would also have an excellent chance to keep the Senate, hold onto a wide margin in the House and enjoy more control of federal government than they have in over a decade.

If they nominate Ted Cruz, Clinton would probably win, the GOP Senate majority would also be in peril and GOP House losses could climb well into the double digits. A Donald Trump nomination would not only make Clinton’s election very likely and raise the odds of a Democratic Senate; it could force down-ballot Republicans to repudiate Trump to survive, increase pressure on a center-right candidate to mount an independent bid and split the GOP asunder.

In other words, if you’re a member of the Republican Party who wants to win in November, it’s basically Rubio or bust. The “Rubio or bust” theory relies on a process of elimination rather than an assessment of his biography, skills or ground game.

There are seven Republican candidates polling above 5 percent in Iowa, New Hampshire or nationally. Three of them — John Kasich, Chris Christie and Jeb Bush — are competing for moderate GOP voters in New Hampshire, but their appeal remains so tepid with conservative Republicans who dominate most other primaries that they lack a plausible path to the nomination.

Percentage of GOP primary voters who ‘could see themselves supporting’ … CANDIDATE CONSERVATIVE GOP VOTERS (61%) MODERATE/LIBERAL GOP VOTERS (38%) ALL GOP PRIMARY VOTERS Ted Cruz 83% 55% 71% Marco Rubio 71 60 67 Donald Trump 72 53 65 Ben Carson 68 46 60 Chris Christie 48 48 48 Jeb Bush 42 43 42 John Kasich 29 26 28 Source: January NBC/WSJ Poll

On the other hand, Trump and Cruz are more popular with conservative Republicans. But either could turn into the most disastrous GOP presidential nominee since 1964. Surveys conducted by NBC News and The Wall Street Journal in December and January tested Clinton against Trump, Cruz and Rubio among “swing” demographic groups — the groups that typically decide who wins key battleground states. Although polls that test general election matchups this far from Election Day (particularly before the nominees have been chosen) haven’t always been predictive, the relative differences between the candidates are telling; the “electability gap” between Rubio and Trump/Cruz was quite obvious:

Which Republican can beat Clinton? HEAD-TO-HEAD POLLING MARGIN VS. CLINTON GROUP RUBIO CRUZ TRUMP White women +10 +4 -4 Independents +7 -1 -9 College-educated whites +6 -3 -11 Seniors (age 65+) +6 +3 -4 Moderates -3 -14 -22 Suburban women -6 -13 -19 Latinos -34 -35 -42 Source: Average of December 2015 and January 2016 NBC/WSJ Polls

Cruz certainly fares better than Trump, outperforming Clinton among two of these seven groups while Trump loses all of them. But unlike Trump and Clinton, Cruz hasn’t been the subject of years-long media coverage, and many voters still don’t know him. Democrats salivate at the prospect of defining him as the architect of the GOP’s government shutdown who personally showed up to cheer Kim Davis’s defiance of the Supreme Court’s same-sex marriage ruling.

Ben Carson wasn’t tested in the NBC/WSJ matchups, but his poor grasp of foreign policy and bizarre statements have written a ready-made script for Democrats to argue that he’s well out of the mainstream and in way over his head.

That leaves Rubio. It’s true that he can sound overly rehearsed, and his machismo can come off as trying too hard. After launching his campaign with a theme of a “new American century,” Rubio has realized that he needs to be more confrontational to woo angry conservatives. But unlike Trump, Carson and Cruz, he hasn’t done or said anything that Democrats could use to flat-out disqualify him in the eyes of swing voters.

If anything, Rubio has plenty of general election upside.

It’s hard to imagine Clinton matching the share of Latino voters that Obama won in 2012, 71 percent, against a Spanish-speaking son of immigrants who supported a bipartisan immigration reform bill. It’s also hard to imagine Clinton matching Obama’s 60 percent among 18-to-29-year-olds against a candidate two decades younger than she is. Finally, unlike in 2012, Democrats wouldn’t have the luxury of portraying the GOP nominee as a corporate robber baron who has never walked in voters’ shoes.

The stark differences between the leading GOP contenders’ appeal in a general election mean that Republican chances in 2016 may not depend so much on Clinton’s popularity. It may be time for Republicans to be more concerned with their own candidates’ electability than Clinton’s favorability, which has slid markedly over the past year.

American Crossroads, a well-funded super PAC guided, in part, by Karl Rove and dedicated to electing a Republican president and Congress in the fall, has largely existed as a stand-in entity to attack Clinton while Republican candidates fight among themselves in the primaries. But its resources might be better spent attacking Trump and Cruz. Astoundingly, Trump has not yet been the target of a sustained negative ad campaign.

Days before the Iowa caucuses, the Republican Party is on the verge of self-destruction. Between now and June, GOP voters will have to decide whether they want to prolong their catharsis or elect a president. Unless Republicans ditch the field altogether and nominate House Speaker Paul Ryan at a contested convention, it’s Rubio or bust.

Listen to the latest episode of the FiveThirtyEight politics podcast.