Tonight's first presidential debate of the 2016 election season could be a cracker - or a complete damp squib.

As has happened so often already during the campaign, it's really all about which Donald Trump turns up on the debate stage in Long Island, New York. If he's got any sense, he'll arrive with the thoughtful, sober persona that has occasionally been on display over the past few weeks.

Being the ludicrous, controversial figure he's been on many occasions over the past 12 months might make for good TV viewing, but it's likely that all those who are impressed by such antics are already firmly in the Trump camp.

Tonight needs to be about appealing to the independents and the undecided voters - and they will need to hear sensible policy debates rather than absurd claims and accusations. So a more presidential display should probably be the order of the day for the mogul. That said, there's a risk in Trump taking that approach.

When he's being controversial, brash and downright offensive, he hogs the limelight. The media loves it, and even those commentators and networks who profess to abhor everything he stands for can't help falling over themselves to focus on the latest Trump outburst, resulting in the Republican enjoying hours of free airtime. So a more reverential tone from Trump tonight could ironically play into the hands of Hillary Clinton, his Democratic rival, who would much prefer a head-to-head on substance rather than style.

Love her or loathe her (and most Americans seem to be in one of those camps), there's no doubt she knows her stuff. If the election were purely about political experience and policy knowledge, she'd have already won this race hands down. So Trump's dilemma going into tonight's debate is which of his two personalities he'll bring with him.Not that Clinton doesn't have her own dilemmas to address.

She hasn't really recovered from her missteps of a couple of weeks ago, when her health became a central campaign issue. National opinion polls have shown there is now not much space between the two candidates - a weekend Washington Post/ABC News poll put them in a virtual dead heat nationwide. Clinton still seems to be edging it in the key battleground states, but it's Trump who has been making the gains there, too. Colorado was a state that Clinton had firmly in her sights, but it's now neck and neck, while her lead in Virginia remains significant but is falling, according to a CBS poll.

The Clinton camp has been furiously spinning the idea that there is greater pressure on her than on Trump During tonight's TV spectacle. This is standard fare for a frontrunner's campaign - the person leading the field always tries to pre-empt these occasions by lowering expectations. And her team is already crying foul of her opponent, claiming that Trump is going to be spouting lies throughout the debate, and urging the media to fact-check everything he says. In other words, Clinton knows the pressure is on her, and she realises that this race is far from over, despite the chattering classes claiming until recently that it was in the bag for the Democratic candidate.

Tonight could be one of those events where a lunatic outburst from Trump undoes much of his recent progress in the polls, or it could cement the perception of Clinton as being shadowy and secretive.

However, don't rule out there being a lot of heat and light, but with little change by the end of it. And, secretly, that might allow both candidates to breathe a sigh of relief.