Selecting the president of the European Commission, the powerful executive branch of the European Union, has always entailed a lot of politicking and bluster, and there’s been no shortage of that in advance of a meeting of European leaders later this week. The difference this time is that Prime Minister David Cameron of Britain is staunchly opposed to the likely pick, Jean-Claude Juncker of Luxembourg, and that is shaping into a potential turning point for the entire European project. Mr. Cameron has a good case, but he should make sure that his looming defeat does not turn into an irrevocable march toward the “Brexit,” a British exit from the European Union.

The issue is not necessarily Mr. Juncker, though some of the British press has painted him as a die-hard European federalist out of touch with the anti-European Union sentiments confirmed in the recent elections to the European Parliament. Mr. Juncker, 59, is indeed committed to European integration, which any president of the European Commission should be, but he is also a seasoned politician who served 18 years as prime minister of Luxembourg.

He comes from the same center-right part of the political spectrum as Mr. Cameron’s Tories, and the Lilliputian grand duchy he comes from is beyond the rivalries of the European powers. Given the mood in Europe, he would most likely promote the reforms sought by Mr. Cameron to make markets more competitive and return some European Union powers back to member states.

Mr. Cameron’s real objection is that Mr. Juncker has been put forward by the European Parliament. The rule is for the European Council, which is composed of the heads of state or government of the 28 union members, to nominate a president on the basis of the European elections, and for the European Parliament to approve or disapprove the candidate by a simple majority. The way it’s being done this time around, Mr. Cameron argues, is in effect a power grab by the Parliament and an irreversible shift of control over the European Union from national governments to Brussels. At a tough meeting on Monday with the president of the European Council, Herman Van Rompuy, Mr. Cameron said that if Mr. Juncker’s name is proposed on Friday, he will break with precedent and demand a vote to show that he is not alone in his opposition.