Leading article: Barack Obama must not underestimate his rival

Mitt Romney has to be held to account for what he would do if elected, not for who he is

With his sweeping victory at this week's Texas primary, the former Governor of Massachusetts, Mitt Romney, has placed his nomination as the Republican presidential candidate beyond doubt. Any excitement about a stand-off at the Republican Convention must now be laid to rest. It will be Mr Romney's name on the ballot alongside that of Barack Obama in November.

Now the campaign begins in earnest – and not just because the Republicans' bruising primary season is at an end, but because any hopes President Obama might have harboured about coasting to a second term should already have been dispelled. Latest polls show the two men running neck and neck. Mr Obama has a fight on his hands, and it is one where incumbency will not of itself be an advantage.

In fact, the opposite could be true. Mr Obama, who might justifiably have adopted George Bush's catchphrase about being "a uniter not a divider" when he came to office, has been a divisive President. The Republicans were deeply split between their fundamentalist Tea Party wing and the rest, but in Congress they closed ranks to be as obstructive as the most cohesive opposition. For Mr Obama, a new, more biddable, Congress is quite as desirable as re-election.

These past two years of often bitter skirmishing in the legislature mean that the early battle lines for the presidential race are already drawn. One argument, which threatens to become even nastier than it already is, relates to banking and big business, and it is one Mr Obama must treat carefully. At issue is Mr Romney's money and how he acquired it – as a successful venture capitalist. The temptation, already apparent, is for Democrats to cast Mr Romney as a ruthless speculator whose fortune was built on the backs of exploited workers.

Being perceived as ideologically anti-business, however, would be a liability for a US politician, whose election – like it or not – depends to a large extent on raising dollars. Mr Romney has to be challenged on what he would actually do as President, for business as for everyone else, not for what he is – a very wealthy man, as many presidents have been.

To wage a positive campaign, Mr Obama has the changing US demography on his side. Young, black and Hispanic voters may not be as enthusiastic supporters as they were four years ago, but they are unlikely to vote Republican. He can offer this constituency relatively good economic figures and an unapologetically liberal social agenda, including wider access to health insurance and express support for gay marriage, that paints Mr Romney into a corner as an arch-conservative. For all the success of the Tea Party, this cast of mind is in decline.

On foreign policy, Mr Obama can extend his appeal to the country's hawks by saying he understands which wars to fight and how to fight them with minimal loss of American lives. He has taken the US out of Iraq and is withdrawing from Afghanistan. Accusations that the US has ducked out of global leadership can be parried with reminders about Osama bin Laden – killed on his watch and on his order.

This does not mean that there is no mileage for Mr Romney in appealing to US patriotism and pledging to safeguard its superpower status. But the voters' appetite for projecting military power has faded, and he risks coming across as a war-monger. That his team speaks in discordant voices shows how tricky charting a distinct foreign policy will be.

But this campaign is just beginning. And for all the talk of US decline and the disputed 2000 election, the US is still the world's dominant power and held up as a stellar example of democracy. Far beyond American shores, it still matters enormously how the campaign is conducted, and – even more – what will be the result.