Leading article: Mr Obama must avoid the trap of complacency The decision by Rick Santorum to suspend his campaign for the Republican presidential nomination leaves the former Governor of Massachusetts, Mitt Romney, clear to become his party's candidate to challenge Barack Obama. The contest that has seemed likely since the start of the primary season - despite a few upsets along the way - has now become a certainty. It will be Obama against Romney in November. This result might have been expected, but the primary season has been costly for the Republicans, and in some senses is not over yet. The cost lies less in the length of the campaign than in the rawness of the battles and the depth of the personal animosities on display. That Mr Santorum merely suspended his candidacy, rather than declaring it over, and chose not to endorse his main rival is evidence of the continued ill feeling. Mr Santorum may have been beaten by the numbers and the money, but he is sticking to his politics - a breach that can only harm the party's prospects. The exit of the third candidate, Newt Gingrich, can now only be a matter of time. But his relative good humour and declining impact make him a benign force in the party compared with Mr Santorum, whose particular brand of conservative conviction politics retains its appeal for many Republicans. The more exotic outliers of the Tea Party movement seemed unusually quiet through the primaries, but that was in part because their concerns were voiced by Mr Santorum. The yawning ideological rift will not be easily bridged. Such persistent tensions aside, early polls consistently forecast that Mr Romney would be a more even match for Mr Obama than any other Republican. And, despite the bruising primary season, that may still be true. Yet the election-year dynamics have been changing, and mostly in Mr Obama's favour. The US economy is starting to look healthier than it did. The killing of the unarmed black teenager, Trayvon Martin, in Florida - which mesmerised America - returns the race issue to the spotlight and tilts the political advantage further towards Mr Obama. Demography is also working for the Democrat, with younger voters more open-minded on race and sexual mores than their parents and grandparents. Mr Romney has his own personal and political baggage: from his Mormonism - which was no bar to his election as governor of Massachusetts 10 years ago, but which appears to have been a liability in the southern primaries - to the extent he minimised the taxes paid on his personal wealth, to the similarities between the healthcare reforms he introduced as state governor and those of Mr Obama's controversial health reforms. Mr Romney may have won the nomination, but he prevailed in a weak field and starts his campaign proper with numerous disadvantages. Half a year, though, is as long a time in US politics as it is anywhere else. And Mr Obama cannot afford to be complacent. He has the advantages of incumbency, but the disadvantages, too, if something seriously untoward occurs on his watch. He will also know that what the most conservative Republicans lack in numbers, they make up for in commitment and perseverance. Mr Romney, for his part, is no political neophyte, and will have behind him all the campaign power that money can buy. In the absence of a plausible third candidate - which cannot even at this stage be completely ruled out - it is to be hoped that Mr Romney can make a real contest of it. It is salutary for any national leader seeking re-election to have to return to the campaign trail and defend his record. With the eyes of the world on its progress, a keenly-fought American election is good for the next President, good for the United States and good for the cause of democracy, too.