Those big TV debate questions: Can Al relax? Will George fluff his lines?

With Al Gore and George W Bush locked in a statistical dead heat, the outcome of this year's US Presidential election could be sealed in 90 minutes of prime-time television tonight.

With Al Gore and George W Bush locked in a statistical dead heat, the outcome of this year's US Presidential election could be sealed in 90 minutes of prime-time television tonight.

The two leading presidential candidates will hold the first of three debates at the University of Massachusetts in Boston this evening, an encounter pundits feverishly trumpet as the High Noon gunfight that will at last establish a clear winner in a nailbitingly close race.

Will Vice-President Gore be able to assert his superior grasp of the issues without coming across as stiff and overbearing? Will Governor Bush be able to project his affable personality without being shown up for a bumbler or a fool?

These are the questions to which an estimated 60 million Americans - including a large number of crucial swing voters - will seek answers as the candidates push against the formal constraints of the debate rules and jockey for position from behind their podiums.

These are also questions that could determine the pattern and pace of the rest of the Presidential race: advisers from both camps have put their candidates' campaign strategies on hold until they can assess the debate's fallout.

The candidates have spent days locked in intense preparation, Mr Gore in Florida and Mr Bush at his ranch in Crawford, Texas. The Vice-President has gathered 13 "ordinary people" to act as a sounding board for his ideas to ensure he neither overwhelms the audience with detail nor comes over as arrogant.

The Texas governor has been immersing himself in policy detail to avoid accusations of being unprepared and unqualified for the most powerful political post on the planet.

The odds favour Mr Gore, if only because of his more accomplished debating record and the gravitas that comes with his much longer experience of public office. While the overall opinion polls show the two candidates neck and neck, Mr Gore is ahead in state-by-state breakdowns of votes for the presidential electoral college. But in an election year that has proved to be remarkably volatile, everything could change with just one slip, or injudicious remark, or even an inability to wrongfoot Mr Bush with sufficient conviction.

"For Bush, the debates may be his only chance to win," said Robert Beckel, who was Walter Mondale's campaign manager in 1984. "For Gore, they are his only chance to lose."

Mr Bush's biggest liability may be his much-remarked inability to maintain a proper grasp of the English language. His propensity to utter "Bushisms", embarrassing, often amusing, manglings of his mother tongue, has grown in the campaign pressure. Last week, he said "more and more of our imports come from overseas".

Earlier, he proudly said Republicans "don't believe in planners and deciders making the decisions on behalf of Americans". When this month's Vanity Fair speculated that he might be an undiagnosed dyslexic, he said he "had never interviewed" the author, Gail Sheehy. He meant she had never interviewed him.

Despite the Democrats' gleeful anticipation of further linguistic pratfalls in tonight's debate, it is perhaps worth remembering Mr Bush is never more dangerous than when he is being underestimated.

When he stood for the Texas governorship in 1994, he stunned everyone by steamrollering the Democratic incumbent, Ann Richards, in their pre-election debate, playing the sweetly reasonable nice guy and making her look imperious and over-aggressive.

Mr Gore, astonishingly aggressive in his primary debates with Bill Bradley this year, will have to watch for his own form of gaffes. Although perfectly at ease with English, he has a habit of over-aggrandising his role in government, claiming credit for the invention of the internet, or for a piece of environmental legislation that was passed two years before he entered Congress in 1976.

Boston is only the first test on the debate treadmill - there are two more in the next fortnight - but in a race where perception has been as important as substance it could well set the tone for the rest.

American Presidential races are three-act dramas, with the primaries coming first, then the conventions, and finally the sprint to election day. Mr Bush came out ahead of Act One, while Mr Gore stole his thunder in Act Two. Tonight, the climactic drums will start rolling.