May and Cairns alike are effectively arguing that the Brexit vote gave them a democratic mandate to limit freedom of movement. Given that that question wasn't even asked, their argument is at the very least open to question. It's interesting to note that some of those who take the same position about freedom of movement in the post-Brexit context feel entirely free to criticise Trump for his edicts on admitting people from selected countries. But it seems to me that he has, if anything, a better democratic mandate for what he's doing than the UK Government has for its position on controlling EU migration. It raises a question about how we determine 'how good' a mandate is; but there's also a wider question about whether, and to what extent, politicians are bound to follow public opinion.
Does Trump have a democratic mandate for rounding up certain categories of immigrant and then deporting them en masse? I think that he probably does. Some aspects of what he wants to do might be contrary to international law, but he's shown himself willing to simply withdraw from any treaty which constrains him. How far can the 'democratic mandate' argument go? Flogging, hanging, public executions in town centres? What about eliminating sections of the population - if it's what the majority wants, why shouldn't the government do it?
I'm using extreme and provocative examples, of course. But once we agree that the electorate do not have an absolute right to demand any policies that take their fancy, once we agree that politicians do not have to follow public opinion on all issues, then where and how do we draw the line? And who draws and enforces it?
I think that Trump has crossed a line, and it's clear that many people agree. But I'm finding it very hard to identify where the line actually lies. And it's precisely because of that difficulty that societies can fall, gradually, one small step at a time, into undemocratic regimes. Of course there's a problem when the much-maligned 'elite' decides that it knows better than the people, and I understand the feelings that that has aroused over the last year or so in particular. But merely following the 'will of the people' brings its own problems - it's far from being the 'solution' as which it's proposed.
I don't really know what the answer is. I think that a written constitution would help - the written constitution of the USA has provided a basis for legal challenge to some of Trump's actions - but it's not a cure-all. Having a political class with enough courage to lead, rather then meekly follow, public opinion is the best starting point that I can think of, but we're moving away from, rather than towards, that position. The prospects are not looking good.