LONDON — Prime Minister Theresa May’s Conservative government in Britain is in turmoil. But the resignations that have rocked it in recent days — even that of Boris Johnson, who was until recently her obsessively ambitious foreign secretary — risk blinding us to a simple truth: The big reason Mrs. May’s party is in so much trouble over Brexit is that it is determined at all costs to end “the free movement of people” that, even for those European countries outside the European Union, is a condition of belonging to the bloc’s single market.

Why are Britain’s Conservatives so set on that course, despite the fact that access to that market is vital to the prosperity of the country they govern? Because promising to “take back control” of their country’s borders gradually became the party’s default response to a challenge that so many of Europe’s center-right parties have been trying to deal with for a decade or more.

The rise of anti-immigrant nationalist insurgencies claiming to represent “the people” against a corrupt and uncaring political establishment has deep economic, political, social and cultural roots. Yet the reaction of the Continent’s mainstream conservative, market-liberal and Christian democratic parties can be boiled down to four fairly shallow, and equally ineffective, approaches. Only if the center-right fully faces up to the fact that they are all dead ends can it begin to come up with better, more creative and probably more combative ways to deal with the challenge it’s facing.

The first approach is to try to ignore the populist radical right — and even treat it as some kind of pariah. That’s essentially what the Christian Democratic Union of Germany and the Moderates in Sweden did for years. In the end, it hasn’t worked.