Credit Rick Santorum with accepting the obvious: he lost. Despite winning 11 primaries, Mr Santorum was always more of an irritant than a plausible contender for the Republican nomination. He surrenders the field having done a meaningful, though not enormous amount of harm to his side.

Mr Santorum made Mitt Romney’s task harder by forcing him to continue spend on the primaries money that could have gone against the general election, by exacerbating doubts about his conservative credentials, and by reminding swing voters that many Republicans remain unfriendly to women, hostile toward the separation of church and state, and too interested in other people’s sex lives.

Mr Romney is none of those things, however, and with Mr Santorum out of the way, he now can get on with the business of challenging Barack Obama. With the primaries unofficially over, the shape of the fall race is fast emerging. Simply stated, Mr Romney is going to run against the Obama economy while Mr Obama runs against Mr Romney himself.

The Republicans will argue that every problem the country faces, from the anemic economic recovery to high petrol prices is the fault of a well-meaning, but inept incumbent. The closest analogue to this campaign might be the one Bill Clinton conducted against the elder George Bush in 1992. Like Mr Clinton in that year, Mr Romney is attempting to exploit the lag between how the economy is and how it feels.

But Mr Romney lacks Mr Clinton’s political skills, to say the least. Also, his case will be a far trickier one to make, because the recovery is further along than it was at a comparable point 20 years ago. In interviews, Mr Romney is forced to concede that the economy is in fact growing, and must try to establish the more complicated proposition that Mr Obama is nonetheless holding back its true potential through excessive intervention. This puts Mr Romney in the sour position of constantly underscoring – and hoping for – bad economic news while foreclosing his own future options in dealing with it.

If Mr Romney’s pitch resembles a Democratic one, Mr Obama appears to be preparing a Republican-style negative assault on his opponent’s character. This is what George W. Bush’s advisor Karl Rove did in 2000, seizing on a few gaffes to cast Al Gore as a serial exaggerator, and in 2004, when he portrayed John Kerry as a slippery flip-flopper. Mr Obama’s advisors David Axelrod and David Plouffe are attempting to define Mr Romney as both a political opportunist who tailors his views to suit the moment and as an out-of-touch plutocrat who can’t relate to ordinary people.

Their chief ally in this effort has been Mr Romney’s mouth, from which have emerged the most extraordinary string of self-defaming comments, ranging from the pleasure he takes in being able to fire people to the fact that his wife has “a couple of Cadillacs.” The most recent blow was a story about Mr Romney’s plan to build an automobile elevator into his newest extravagant vacation home. Mr Obama will not attack Mr Romney’s wealth directly. Instead, he will portray his opponent’s policies as designed to benefit the rich at the expense of the middle class, as with his attack last week on the budget plan Mr Romney supports as “thinly veiled social Darwinism”.

The race begins with Mr Obama 5 to 10 percentage points ahead in national polls, better funded, and ahead in grass-roots organising in several of the key battleground states. But while the President starts with the upper hand, this is likely to be a close, hard-fought contest. One risk is that the personal assault Democrats level against Mr Romney backfires against Mr Obama, defining him as a thuggish, “Chicago-style” politician. At the very least, the President’s angrier, populist turn will cut against his efforts to convey a sunny, optimistic outlook. Another hazard for Mr Obama is the new vehicle known as the Super PAC, which allows the rich to contribute unlimited sums to technically “independent” efforts. Spurred by the billionaire Koch Brothers, Republicans expect to raise in excess of $400 million for advertising against Mr Obama.

The biggest risk to Mr Obama remains unchanged. With 209 days to go until the election, it is the US economy itself. A few more weak reports from the Labor Department or a replay of last summer’s recession scare could turn to mockery the President’s message that things are getting better. His modest lead is bound up with a still-tenuous recovery.