Introducing his running mate against the backdrop of the USS Wisconsin on Saturday, Mitt Romney flubbed his easiest line: “Join me in welcoming the next president of the United States.” There is no way to avoid reading this as a Freudian slip. Mr Romney’s chief problem as a candidate has been his substantive vacuity, his failure to stand for much beyond flexibility itself. In choosing Paul Ryan, he opted to outsource the content of his campaign to his opposite: a principled, conservative ideas man. Mr Ryan is now the head of the Republican ticket, Mr Romney the body.

Given the options he had left himself, this was probably the best choice for Mr Romney to make.

Mr Ryan stands for a clear proposition – the radical scaling back of the federal government’s social commitments – and through his pick, Mr Romney now represents that as well. Usually, a vice-presidential candidate scrambles to fall into line with the top of the ticket. In this case, it is Mr Romney who will, not for the first time, adjust his views. Instead of attacking Barack Obama for cutting Medicare, Mr Romney must now charge him, as Mr Ryan does, with not cutting it enough.