shows British Prime Minister David Cameron (R) and then-London Mayor Boris Johnson (L) in 2012. The two Conservatives are now at odds over Brexit. WILL OLIVER/AFP/Getty Images

While Americans are grappling with the rise of Donald Trump and what it means for the future of American politics, the British are set to vote on what may be an even more momentous issue: “Brexit” – whether Britain should stay in the European Union, or leave. There will be a national referendum on the subject on June 23.

I. How the Brexit Debate Cuts Across Ideological Lines.

An interesting aspect of the Brexit debate is the way it cuts across ideological lines. Conservative Prime Minister David Cameron opposes Brexit. But many in his party – including former London Mayor Boris Johnson (whom many see as a possible successor to Cameron) are on the other side. On the left, the leadership of the Labor Party opposes Brexit. But there is still significant support for the “Leave” campaign on the left, particularly among far-left figures, such as George Galloway. Jeremy Corbyn, the very left-wing leader of the Labor Party, officially opposes Brexit. But his defense of the EU seems tepid to Europhiles in his own party, and some commentators think he may even be a closet supporter of Brexit.

The debate over Brexit has also divided libertarians and other free market advocates. Mark Littlewood, the Director General of the Institute of Economic Affairs – Britain’s leading free market think thank – has published a powerful op ed retracting his previous support for the EU, and endorsing Brexit. But some other libertarians take the opposite position.

Pro-free market Brexit supporters hope that Britain can pursue more free market policies if freed from the regulatory grip of Brussels. By contrast, left-wing Brexit advocates have the exact opposite expectations. They think that Britain would enact more economic regulation and build a larger welfare state if freed from the EU, which they see as a “tool for multinationals.” These widely divergent expectations can’t both be right. If Britain does leave the EU, at least some of the supporters of Brexit will end up badly disappointed.

At the very least, these contrasting predictions suggest that there is a great deal of uncertainty about the consequences of Brexit. Much depends on what policies Britain chooses to pursue in the aftermath, and on how the rump EU reacts to them. Much also depends on what the EU and Britain might do if Brexit is defeated. The European Union has gone through some very difficult crises in recent years, and it is hard to say how it will develop in the future.

Like most other federalism and international relations scholars, I am struck by the fact that the EU is in so many ways different from other political institutions that exist today, or in the recent past. It is neither a fully sovereign federal state like the US or Canada, nor a conventional alliance or international organization, like NATO or the UN. That makes it very difficult to predict its future course, since there is so little relevant comparative evidence. Such a unique institution could end up following a path that not even the most sophisticated experts can foresee.

Another source of uncertainty is that British withdrawal from the EU could potentially lead Scotland to secede from from the United Kingdom. The consequences of that are also highly uncertain, for reasons I outlined during the runup to the 2014 Scottish secession referendum.

II. A (Very) Tentative Case Against Brexit.

As a libertarian and free market advocate, I tentatively lean against Brexit, siding with the pro-EU libertarians against their critics. For all its genuine flaws, the European Union (and the European Community before it) has two great achievements to its credit: establishing free trade and free migration over a vast area. That has made many millions of people freer and vastly better off than they might be otherwise. Brexit would significantly imperil both accomplishments.

Some British free market advocates contend that a post-Brexit Britain could simply sign bilateral agreements on trade and migration with the EU, thereby preserving the beneficial aspects of the status quo, while freeing Britain of EU-imposed regulations. This could indeed happen. But there is reason for skepticism. In the aftermath of Brexit, nationalist public opinion, interest group pressure, and other forces could easily undermine any potential agreement, or at least make the flow of people and goods far less free than it was before. Especially when it comes to migration, many Brexit advocates regard increased barriers between nations as a feature rather than a bug. It is one of the main reasons why they want to leave the EU in the first place. A successful Brexit campaign would probably strengthen these forces, and make it harder to reestablish free trade and migration, even if the European Union were willing to play ball (which is by no means certain).

Free trade and migration are hugely important for both freedom and human welfare. Absent free trade with the EU, millions of British consumers would have to pay higher prices for important goods (some EU consumers would also be harmed). Absent free migration, hundreds of thousands of immigrants might be consigned to lifelong poverty in their countries of origin, or at least in European nations where rigid labor market restrictions make it difficult for immigrants to get ahead than it is in Britain. It would take a truly enormous reduction in economic regulation on other fronts to outweigh that. And it is far from clear that a post-Brexit Britain would really regulate its economy significantly less than it does now. My reasons for tentatively opposing Brexit could qualify as reasons for supporting it, for adherents of some ideologies. But they count heavily against Brexit if you are a libertarian, a free market advocate, or are concerned with increasing happiness and well-being.

Although libertarians, conventional conservatives, and the left are all divided over Brexit, one important group is not: xenophobic nationalists who combine support for a large activist state with vehement opposition to free trade and immigration. These forces are represented in Britain by the anti-EU UKIP Party, and in the US by Donald Trump and his hard-core supporters. As economist Scott Sumner emphasizes, one notable reason to oppose Brexit is that a British withdrawal from the EU would strengthen these forces, which are inimical to everything that libertarians (and other cosmpolitan liberals) stand for. Brexit would give nationalists new political momentum, and might also exacerbate economic turmoil that could further redound to their advantage.

It would be a mistake to oppose Brexit solely because xenophobic nationalists support it. That would be succumbing to Eugene Volokh’s “Reverse Mussolini Fallacy.” The fact that Mussolini wanted to make the trains run on time doesn’t mean that making the trains run on time is a bad thing. Similarly, the fact that various bad and severely misguided people support Brexit doesn’t necessarily mean Brexit itself is bad.

Nevertheless, it is significant that Brexit is likely to strengthen dangerous nationalist forces in Britain, and perhaps elsewhere. As Sumner notes, that increase in their power could easily have negative effects on other issues. Brexit could also potentially strengthen the far left, who are generally more likely to oppose the EU than more moderate left of center forces. Moreover, in a situation of great uncertainty, the fact that some of the very worst political forces in Europe are arrayed on one side is at least worth pondering. If the consequences of making the trains run on time were highly uncertain, but we knew that fascists are in favor of it, that should incline us against it, other things equal. It would not be a definitive consideration, by any means, but still one we should take account of. It is only because the evidence here is otherwise clear and unequivocal that it makes sense to completely discount what the fascists think.

Sumner also emphasizes that rejecting Brexit does not mean that the Britain must support all of the EU’s dubious regulatory policies, and that it could potentially do considerable good by opposing them from within. He notes that a rejection of Brexit now does not necessarily preclude adopting it at some point in the future, if – for example – the European Union adopts a common fiscal policy.

Like Sumner, I tentatively lean against Brexit. But I recognize that there are strong arguments on both sides, and a great deal of uncertainty. British voters have a genuinely difficult decision to make this June.