Nicolas Sarkozy faces Francois Hollande in a 'moment of truth' television debate on Wednesday that could be the French president's last chance to avoid defeat at the hands of his Socialist challenger in Sunday's election runoff.

The conservative president and his centre-left rival have been at each other's throats for months, with Sarkozy accusing Hollande of being incompetent and a liar, and Hollande calling the incumbent a 'failed president' and 'a nasty piece of work'.

Trailing Hollande despite an aggressive campaign, Sarkozy will throw everything into a two and a half hour verbal duel with the Socialist who, despite his bland manner, is a nimble debater himself.

"It's not a contest of words, it's a moment of truth," Sarkozy told journalists this week, as an aide let slip to Reuters that the incumbent will spend most of Wednesday holed up at home preparing.

"Sarkozy needs to swing 1.5 million people to his side. It won't be easy but that doesn't mean it's impossible," Bernard Sananes, head of the CSA polling institute, told BFM TV.

Blamed for the country's economic problems and widely disliked because of his brash personal style, Sarkozy is the most unpopular president to run for re-election and the first in modern history to lose a first-round vote to a challenger.

He began campaigning weeks before Hollande, vowing to boost industrial competitiveness, hold referendums on policy, crack down on tax exiles and make the unemployed retrain if they wanted to receive benefits.

More recently, he vowed to cut immigration, said Hollande would drive France towards economic catastrophe and threatened to pull out of Europe's border-free Schengen zone unless European Union borders are strengthened.

Hollande, who enjoys a six to 10 point lead for the May 6 runoff, denied Sarkozy's taunt that he was a 'dodger' for turning down a challenge to hold two extra debates.

"Let him come to the debate this evening and pose his questions," Hollande told BFM TV.

"Frankly, the only question which needs to be asked is, do the French want the same failed policies of the last five years? Rising unemployment, weak growth, debts and deficits."

Sarkozy received a setback on Tuesday when far-right leader Marine Le Pen - who shocked France by coming third with 18 per cent in the first round - refused to endorse Sarkozy. She said she would cast a blank vote and told her 6.5 million voters to make their own choice.

A TNX Sofres poll published on Wednesday found that 37 per cent of people were in agreement with the National Front's positions on immigration and national identity, up 6 points from January and the highest level since 1984. Fifty-two per cent of those questioned said there were too many immigrants in France.

The debate comes as the race has been clouded by mudslinging and sleaze allegations, with Sarkozy filing a lawsuit against a news website that alleged Muammar Gaddafi's government sought to fund his 2007 campaign.

POLITICAL BRAWLER

A formidable political brawler, Sarkozy is convinced he can swing things in his favor on Wednesday evening by portraying Hollande as lacking in experience and economic credibility.

A daily opinion poll by Ifop showed a narrowing of Hollande's lead to seven points on Tuesday from 10 points the day after the first-round vote.

"Sarkozy is very combative, very pugnacious. He can be quite hard with his interlocutors and Hollande has to avoid being seen to be browbeaten," said Christopher Bickerton, an associate professor of international relations at the Sciences Po university.

Twenty TV cameras will scrutinize the two rivals from every angle as they sit 2.5 meters (8 feet) apart across a table.

The two sides have agreed on logistical details down to the temperature of the TV studio - between 19 and 20 degrees Celsius (66-68 Fahrenheit) and chairs that can be adjusted for height.

The debate starts at 9 p.m. (1900 GMT) and, monitored by a large wall clock, must not run over 2 hours and 30 minutes.

Sarkozy's supporters see the debate, expected to be watched by around one-third of France's 63 million people, as his last chance to land a killer blow on his opponent.

"Hollande will be down on his knees," Jean-Pierre LeGrand, 63, a retiree said at a rally of his conservative UMP party in Paris on Tuesday.

Sarkozy and Hollande both deny having trained with coaches or held dress rehearsals with stand-in sparring partners. They have come face to face several times in the past, most notably in a 1999 debate on Europe.

"I have no coach, just myself," Hollande said on Tuesday. "It's not a boxing match or a wrestling match."

Sarkozy - who has betrayed the stress he is under by seeming irritable and aggressive in some recent appearances - intends to 'flush' the seemingly unflappable Hollande 'out of the woods', an aide told French media this week.

The only debate considered to have swung a tight French election was in 1974, when Valery Giscard d'Estaing emerged stronger for hitting Francois Mitterrand with the snub: "You do not have the monopoly of the heart."

Mitterrand beat Giscard d'Estaing seven years later and delivered a crushing line in a 1988 debate against conservative challenger Jacques Chirac, then his prime minister.

When Chirac, a few years his junior, said: "Allow me to say that this evening, I am not the prime minister and you are not the president, we are two equal candidates. You will therefore permit me to call you Mr. Mitterrand," the Socialist replied jauntily: "But you are quite right, Mr. Prime Minister."