Desperate to avert the impending reality that Donald Trump will be named the Republican Party’s presidential nominee, some party leaders have been agitating to block him at the GOP convention in Cleveland. Having failed to get their act together to derail Trump at the ballot box, they now hope against hope that they can turn the convention from the traditional coronation into that rare, political unicorn: the contested convention.

No less a figure than former Republican presidential nominee Mitt Romney laid out a scenario last week where Trump could be denied the nomination at the convention. It requires the candidates remaining in the race to band together and Republican primary voters to begin to cast their vote strategically in favor of whoever can block Trump. He urged Sen. Marco Rubio (R-FL), Ohio Gov. John Kasich and Sen. Ted Cruz (R-TX) to put ego aside and work toward one goal: stopping Trump.

“Given the current delegate selection process, that means that I’d vote for Marco Rubio in Florida and for John Kasich in Ohio, and for Ted Cruz or whoever has the best chance to beat Mr. Trump in a given state,” Romney said. He reiterated again Friday that this was a scenario where he believed Trump could be stopped, while Kasich told attendees at a conservative confab that a contested convention is where this race is headed.

Political scientists, political junkies and political reporters have dreamed about contested conventions for years. Now they are in the grips of the contested convention fantasy made suddenly real by Romney. But experts on the convention process and the complicated politics surrounding such a move argue it will be exceedingly difficult to stop Trump at the convention — and gravely damaging to Republican electoral prospects in the fall and perhaps beyond, even if it is successful in preventing Trump from the securing the nomination.

Even if Trump arrived in Cleveland shy of the 1,237 delegates needed to win the nomination outright (and that is a big assumption), staging an organized stop Trump revolt at the convention assumes that Republican delegates – many of whom are passionate Trump fans – would turn against him and rally around a single alternative candidate even as the party is utterly divided.

Ryan Williams, a former spokesman for Mitt Romney’s 2012 campaign, conceded the chances of stopping Trump in Cleveland are slim. “Even an effort for a candidate who had this thing locked up was a handful,” Williams said, referring to Romney’s effort in 2012 to lock down the nomination and avoid any funny business at the convention.

“I don’t know how the heck you would approach this kind of convention,” Williams said about the complications of stopping Trump at a contested convention. “I can see denying Trump that number, but I think it’s going to be a very difficult task to change enough people’s minds on the second, third, fourth or fifth ballot to get to another candidate. “

There a number of challenges facing party elites currently embracing this tactic. For one, they will first have to convince primary voters – as Romney tried to in his speech last week – to chose their non-Trump candidates strategically in order to deprive him of the 1,237 delegates to win outright.

“I think it will be very difficult, and the reason I think it will be very difficult is firstly, you have to convince voters. Voters are voters. They’re independently minded,” says Richard E. Berg-Andersson, a political analyst for “The Green Papers,” a blog that tracks the nomination process. “Trying to say, ‘We should try to encourage voters to vote for Kasich even if they otherwise might vote for Cruz or Rubio instead,’ it’s kind of like herding cats.”

But even if voters handed the anti-Trump forces the conditions for contested convention, there are major political risks to using a convention fight to stop Trump, especially if he heads to Cleveland with a substantial plurality of support.

The Republican Party would have to make a political calculation and collectively decide that unseating Trump – which could require changing convention rules at the beginning of the nominating convention – was more important to the party’s brand and future than winning in the 2016 general election. Presumably, any candidate that emerges from a chaotic, contested convention would be bruised from the floor fight, which could include multiple rounds of voting. Not to mention, stirring up a tide to overturn Trump after a primary election cycle where Trump built a movement in his favor would alienate many Republican voters.

“If they want to derail Trump at the convention, they certainly have the ability to. It complicates things for winning in November,” said A.J. Spiker, the former Iowa state GOP chair who worked on the Rand Paul campaign. “If they change the rules, there will be a backlash.”

Rule 40 is a prime example of the type of obstacles Trump foes will face in trying to engineer a convention fight to thwart him. The rule — molded to protect Mitt Romney in 2012 after John McCain’s nomination in 2008 was threatened by some unruly Ron Paul delegates — stipulates that a candidate must have won a majority of the delegates in at least eight states to be a nominee. Trump currently has the best shot at meeting that threshold, having won a majority of delegates in five states. The next closest candidate — Cruz, at three states — is by no means the establishment’s favored alternative to Trump. If he nor any of the other non-Trump candidates can meet the threshold, the rule will have to be changed to open the door for one of them be an eventual nominee, and without a clear nominee, pushing through the rule change will be that much more difficult.

“If the Rules Committee is dysfunctional going into the convention, then things we’ve been talking about — what rules stay in and what get thrown out — are not even organizable,” Berg-Andersson said.

The next obstacle for leaders hoping for a contested convention is convincing once-bound delegates to rally behind an alternative to Trump on a subsequent ballot. If nothing changes, and Rubio, Kasich and Cruz are all still in the race come July, why would an alternative to Trump be any clearer than it is today? The party has not been able to rally around a single alternative candidate yet.

“The various campaigns are going to try to get delegates to switch, and I don’t know how they do that,” Berg-Andersson said. “You could go ballot after ballot after ballot.”

Taking a further step back, it’s not clear where the resources would come from to launch a full-fledged stop Trump effort involving a contested convention. The Koch brothers have said they won’t do it. The billionaire Ricketts family has spent a few million going after Trump, but it pales next to the tens of millions spent by the other candidates and their PACs to block Trump and win the nomination for themselves. Jeb Bush alone spent well more than $100 million only to flame out.

Berg-Andersson noted that, if multiple ballots pushed the convention into the weekend, that would mean hotel rooms and other accommodations would need to be paid for to keep the delegates — and press and all other interested parties — in Cleveland.

“You are going to have a big pressure from outside and inside the convention to wrap things up,” he said.