Many will say that American political conventions do not matter any more. To be sure, the days when delegates had to hold 103 ballots to agree a presidential nominee (the Democrats in 1924) are long gone. The last time there was any genuine suspense over a convention's outcome was in 1976, when Ronald Reagan came close to snatching the Republican nomination from the man in the White House, Gerald Ford. That Mitt Romney would carry the party's standard against Barack Obama this autumn has been certain since April and never in much doubt even before that. These days, conventions amount to four-day-long party political broadcasts, choreographed down to the last second, precisely in order to avoid any unscripted surprise.

And so it will be for Republicans when they gather (Hurricane Isaac permitting) for the ritual foot-stomping and schmoozing in Tampa, Florida. But for Mitt Romney, thanks to his own shortcomings as a candidate since he secured the nomination, this convention is hugely important.

When Americans vote for their president, they vote for someone they like to think they know. But this Republican candidate has seemed unable to tell them who he is. Since politics abhors a vacuum, that job has been gleefully performed by the Obama campaign. And the picture it has painted is not pretty: of Romney the callous and shifty private-equity operator, bent only on enriching himself and his cronies, who pays next to no taxes – a US version of the Vicar of Bray, whose views shift as the political moment dictates.

That failure is why he trails in the polls. The feeble US recovery, and the attendant high unemployment rate, makes Mr Obama vulnerable. But Mr Romney's listless performance has allowed the President to keep the argument off the economy. Mr Romney's choice of Paul Ryan as running mate provided a small boost, but Mr Romney still lags in most of the eight or nine states that will determine the result. If the election were held today, there can be little doubt he would lose.

Tampa offers his best, and perhaps last, chance to introduce himself to his countrymen. The networks may be cutting prime-time coverage to one hour a night, but the convention will dominate newspaper headlines and the political blogosphere. This will be Mr Romney's show. His task is not to make Americans fall in love with him, as they fell in love with Mr Obama in 2008. Mr Romney is not that sort of politician. Even if he were not a teetotaller, he would never win the presidential vote for the "candidate I'd most like to have a beer with".

His goal is more modest, but certainly attainable. This is a downbeat and exceptionally nasty election year. Both candidates prefer to tell lies about each other, rather than the truth about what needs to be done, about the tax and spending compromises that will have to be struck to bring down America's debt and cope with the financial challenges posed by an ageing population. Tampa offers a platform from which to tell those truths. Mr Romney needs to win Americans' trust; not their hearts, but their heads. He must persuade them that, as a successful businessman and competent former Republican Governor of Democratic-leaning Massachusetts, he has the skills needed to put his country back on course.

It is a great deal to ask, and the latest distractions caused by a Republican Senate candidate's crass remarks about rape and abortion have not helped. But Mr Romney can draw heart from Bill Clinton's triumphant convention in 1992. It, too, was minutely choreographed, but it transformed the country's view of the candidate. Yes, indeed, conventions can still matter.