The run-off debate is traditionally a pivotal moment in French presidential elections Eric Feferberg. AFP
Macron will go head-to-head with Le Pen late on Wednesday in their only direct clash ahead of Sunday's second-round vote, an encounter set to be watched by millions of French.
Some polls are predicting a lead of around 10 points for Macron over Le Pen in the run-off but undecided voters and abstentions could yet swing the figures.
Both candidates are particularly keen to woo the electorate of hard-left candidate Jean-Luc Melenchon, who finished a strong third in the first round.
The vote will mark the closest the far right has been to seizing the Elysee presidential palace. Marine Le Pen's father Jean-Marie was crushed by Jacques Chirac in the 2002 run-off election and she was easily beaten by Macron in 2017.
Opinion polls currently put the centrist Macron at 53 to 56 percent in the run-off against 44 to 47 percent for Le Pen -- a much tighter finish than five years ago, when Macron carried the vote with 66 percent.
"The game isn't over yet and we certainly can't draw conclusions ... that this election, this match, is already decided," Prime Minister Jean Castex told France Inter radio.
"We have to convince the French that Emmanuel Macron's programmes are the best for France and for them," he said. He added that if Macron won, his government would resign to give the ruling party new impetus ahead of legislative elections in June.
'Either could win'
Castex's predecessor as prime minister, Edouard Philippe, mayor of the northern city of Le Havre and a heavyweight centre-right backer of Macron, said nothing could be taken for granted due to the numerous "unknowns" hanging over the election and, above all, abstentions.
He told Le Figaro newspaper on Monday that the so-called Republican front -- which in past elections had seen French of all political stripes line up against the far right -- "was no longer a natural reflex".
"Right now, either candidate could win," added another ally, Francois Bayrou, the leader of the pro-Macron Modem party.
Macron had himself made clear he does not see the election as being in the bag, reminding voters of the political earthquakes of 2016 when Britons voted to leave the EU and Americans put Donald Trump in the White House.
"Think back to what British citizens were saying hours before the Brexit (referendum) or in the US before the Trump vote came: 'I'm not going (to vote). What's the point?' I can tell you that the next day they regretted it," Macron told France 5 television.
"So if you want to avoid the unthinkable or something that revolts you, choose for yourself," he said.
'Kick in the backside'
The run-off debate is traditionally a pivotal moment in French presidential elections and will be broadcast live on national television from 1900 GMT on Wednesday.
For Le Pen, it represents a final chance to haul back ground in the polls and convince France she has moderated her anti-immigration party into a mainstream force.
Macron is likely to seek to portray her as a dangerous extremist who cannot be trusted on foreign policy -- especially after Russia's invasion of Ukraine, due to her past close ties with President Vladimir Putin.
Le Pen was reportedly spending all Tuesday with her closest aides to rehearse the debate, with no trips on the agenda, to avoid the fumbling and missteps that marked her performance in 2017.
"For me, failure is sometimes a kick in the backside," she told TF1 TV, again acknowledging that she had not been up to scratch in 2017.
She has vowed to ban the wearing of the Islamic headscarf in public but close lieutenants have rowed back on this somewhat, with her spokesman Sebastien Chenu admitting on BFM television that it was a "complex problem".