The Republican tax plan will soon enter American law. The bill passed the Senate in the early hours of Saturday, and will now be reconciled to the not-too-different House version, readying it for the presidential pen. The vote fell strictly on party lines amid an ugly frenzy of last-second amendments scribbled in the margins. Even the senators did not know precisely what they were voting on. So it is worth taking a step back to consider, in broad outline, the changes that the plan will bring.

These fall into three categories: big tax cuts for business, which will significantly increase the budget deficit; a reshuffling of individual tax rates, which will have a much more limited net effect; and several halfhearted efforts at genuine reform.

Economically, the most important question is the effect on growth and investment of cutting the headline corporate tax rate from 35 per cent to 20 per cent. There are few certainties here. One is that the cut will increase the deficit by a large amount — at least $1tn — over a decade. The idea that the cut would pay for itself has modest support in theory and none in history. It is also certain that the cut will produce some growth: it is if nothing else an injection of cash into the economy. The question is whether the growth will be worth the cost. The battle lines on this issue are drawn. Only experience will resolve the debate conclusively.