The weather may well decide how long the Republican convention in Florida will be. Mitt Romney, however, will dictate how successful it will be.

Delegates arriving in Tampa are aware that the winds and rain from Tropical Storm Isaac may shorten their gathering but, even if it is truncated, the important business will be completed and Mitt Romney will leave as the official Republican nominee for President.

The conventions are now so well managed there is little opportunity for a surprise such as Ronald Reagan seeming to go along with the idea of bringing back Jerry Ford in 1980 to run as vice-president, only to name George H.W. Bush as his running mate in a post-midnight news conference. And the carefully stage-managed procession of speakers removes the drama of someone going off script in front of a television audience of millions, causing embarrassment to the candidate or even fights on the floor among the delegates.

The conventions are now little more than highly organised infomercials, a chance for the party to dominate the media, take up valuable television prime time and introduce the candidate to the country.

That's why this convention is so important for Mitt Romney. Given the state of the economy, this is an election that many argue is there for the Republicans to lose. While opinion polls suggest a growing number of voters believe he may be better qualified to handle the economy, most people also have an unfavourable view of the former Massachusetts Governor, though. They see him as stiff and slightly disengaged, with no idea of the worries and struggles of ordinary people.

Remember this was a candidate who offered a $10,000 bet to rival Rick Perry during a Republican primary debate.

So one of the main goals of the convention is to humanise Mitt Romney. Each of the speakers has been chosen to make the best possible case. His wife, Anne, will talk about her husband of 43 years, of how he stood by her during her battle with multiple sclerosis. She will speak also of their family of five sons and 18 grandchildren, who we can expect to be prominent throughout the duration of the convention. People linked to the Mormon church will take to the podium, and there will appearances by former Olympians to remind people how Mitt Romney saved the 2002 Winter Games in Salt Lake City from scandal and potential disaster. And there will speeches from Paul Ryan, his vice presidential pick,  and party favourite and early Romney supporter New Jersey Governor Chris Christie.

And then there is the speech from the man himself. This may be the first time many voters actively engage with the presidential race. For reporters, it has dominated our work and our thoughts since October last year. For 'real' people, the summer is almost over and now they may turn more of their attention to who should be running the country. This will give Mitt Romney a chance to define himself, to present a picture different from the sweep of negative ads that describe him essentially as a business vampire, sucking jobs and money from companies while making himself richer.

He has to move away from the divisive social issues and concentrate on the area where Barack Obama is most vulnerable: jobs and the economy.

And he has to realise that this is the most important hour of his political life.

The convention speech remains one of the key tests of readiness for the top job in American politics, a screen test in front of an estimated audience of 40 million people.

Bill Clinton's opinion poll rating shot up 16 points after his convention speech in 1992. Four years earlier, George H.W. Bush made an emotional speech which helped him connect with voters in a way he never had before.

Mitt Romney is not an inspiring orator. He doesn't lift and move the crowd the way others can and do. He may not leave people believing he's the sort of guy they can share a meal with or sit around and chat through the day. What he has to convince voters of, though, is that he's trustworthy, competent and perhaps better than the other guy. And he has to convince those who haven't yet made up their mind that he can be President of the United States.

Follow Alan throughout the convention on Twitter: @AlanFisher