If Donald Trump wins the Republican nomination for president, any outcome after that could be catastrophic for his party.
Win the presidency, and he might actually try to implement policies – a border wall with Mexico, tearing up NAFTA, banning Muslims from entering the country and rounding up illegal immigrants – beyond anything the most radical GOP members could have imagined.
Or, more likely, he hands the election to the Democrats and leaves his own party a flaming wreck.
No wonder, then, that much of that party is mobilizing to try to stop him. Prominent Republicans are lining up to publicly slag him.
And a Republican Super Pac, Our Principles, is running anti-Trump attack ads. One spot rounds up Mr. Trump's racial comments – in one clip, he implies President Barack Obama only got into law school because of his race; in another, he refuses to condemn the Ku Klux Klan – and suggesting these will all be used against the party by Democrats.
Derailing Mr. Trump's candidacy is theoretically possible. He needs 1,237 delegates to clinch the nomination, and so far has a little over 300 with 35 states still left to vote.
The trouble is that he holds a big lead in the polls – 16 per cent, according to the Real Clear Politics average – and the clock is ticking. Fifteen states hold primaries in the next two weeks. Crucially some of those states, including big prizes Florida and Ohio, award all delegates to the winner, allowing Mr. Trump to run up his total even with narrow victories.
What's more, the Republican field is still splintered. Neither Texas Senator Ted Cruz nor Florida Senator Marco Rubio has been able to pull far enough ahead to become Mr. Trump's default challenger.
Even if the field narrows further in the next two weeks, there is no guarantee Mr. Trump would lose: not all of Mr. Cruz's supporters would go to Mr. Rubio, or vice-versa.
But the stakes are high enough for some Republicans to at least try. Mr. Trump has dominated this election season for months and the coming weeks of the GOP race will be defined by civil war over his run to the nomination.
In a speech at his election night party after the Texas primary, Ted Cruz told supporters nominating Mr. Trump "would be a disaster for Republicans, for conservatives and for the nation."
Marco Rubio, for his part, told CNN: "If we nominate Donald Trump, it will be the end of the modern Republican Party."
Such declarations, coming from the men who want to defeat Mr. Trump for the nomination, might be dismissed as campaign hyperbole. But in this case, they may actually be true.
Despite the consensus among the Republican establishment, the big question of how to stop Trump has no easy answer
The biggest problem facing a "stop Trump" movement is the lack of a consensus challenger.
Texas Senator Ted Cruz is running second, with wins in his home state, Oklahoma, Alaska and Iowa. But he is almost as unpalatable to the Republican establishment as Mr. Trump.
A smooth-talking ideological purist, Mr. Cruz has driven his own party crazy during his three years in the Senate with his constant obstructionism. On more than one occasion, Mr. Cruz pushed to shut down the federal government to protest against Obamacare and funding for Planned Parenthood, then accused his fellow GOP senators of rolling over for the Democrats when they wouldn't go along with his plans.
Meanwhile, third-running Florida Senator Marco Rubio would seem like an obvious anti-Trump choice. He is youthful, Latino, hails from a swing state and is politically pragmatic. But his campaign is stuck in third gear: He won Minnesota on Tuesday and placed a strong second to Mr. Trump in Virginia, but fell far short elsewhere.
Complicating matters is Ohio Governor John Kasich. Despite having so far failed to win a single state, he is likely to stay in the contest until his home state's primary March 15.
At some point, voters and party activists will have to agree on a single horse to bet on if they want to stop Mr. Trump from winning the race.
As Republican voters cast their ballots on Super Tuesday, an anti-Trump SuperPac held a conference call with deep-pocketed donors to pitch them on funding efforts to stop him.
The talks, reported by CNN and The New York Times, citing confidential sources, included about 50 well-heeled Republicans, including Hewlett-Packard CEO Meg Whitman and Rubio operative Paul Singer.
Even if a well-funded anti-Trump attack machine gets under way, it's an open question what the effect would be. For a candidate who thrives on portraying himself as a maverick against the world, the existence of an organized, establishment effort to stop him might play into Mr. Trump's hands.
But anecdotally, the current attack lines against Mr. Trump proffered by the Cruz and Rubio campaigns – that he is insincere, having reversed himself on abortion and previously expressed admiration for Ms. Clinton – have resonated with Republican voters. So it's possible a well-funded effort to drill those into the public consciousness even further could pay dividends.
So far, gaffes that would have torpedoed any other candidate – attacking Senator John McCain for being a prisoner of war in Vietnam, for instance – have had little effect on Mr. Trump's popularity.
But there have been signs that some blunders may finally go too far. After coming under fire for failing to denounce the Ku Klux Klan during an interview last weekend (former KKK leader David Duke has endorsed Mr. Trump), the billionaire went into damage-control mode. He blamed a "bad earpiece" which he said made him fail to hear the interview question and said he would "disavow" the KKK.
And it need not be a gaffe that derails him nationally, either: because of the state-by-state nature of the contest, a stumble that loses him key states – vote-rich Florida and Ohio, for instance – could cost him big in the delegate count.
There is a chance Mr. Trump will fail to secure more than half the delegates to the Republican National Convention in Cleveland in July. This would set up a brokered convention.
In such a scenario, other candidates could team up and pool their delegates in a bid to vault a second – or third-place candidate ahead of Mr. Trump.
This would be an extraordinary event in the United States, which has avoided the proverbial "smoke-filled-room" method of choosing a nominee since the primary system became dominant in the 1970s.
The aversion to a brokered convention is two-fold. For one, it could be perceived as undemocratic for backdoor deal-making to anoint one of Mr. Trump's rivals. For another, the sooner a nomination is decided, the more time the party has to campaign for the general election. Pushing the decision to July could present its own problems to the GOP.
But if desperate times call for desperate measures, this may be the GOP establishment's Plan Z.