Emmanuel Macron and Marine Le Pen girded for a televised debate Wednesday that is likely to prove the climax of this year's turbulent French presidential campaign, with millions of votes potentially up for grabs just four days before ballot casting begins.

The centrist incumbent and his far-right rival will trade blows starting at 9:00 pm (1900 GMT), in a rematch of their 2017 face-off that was widely seen as disastrous for Le Pen.

But this time Macron will not be the ambitious outsider making his first run at public office -- he will have a five-year record to defend against a candidate who has softened her extremist edges to present a more mainstream image.

Recent polls give Macron the advantage, at 53 to 56 percent against 44 to 47 percent for Le Pen, who is making her third run at the presidency, though analysts say low participation could still sharply sway the final result.

Turnout in the first round of voting was just 74 percent, meaning one in four eligible voters stayed home, a pool that both candidates are eager to motivate.

In addition, the fiery hard-left candidate Jean-Luc Melenchon scored nearly 22 percent in the first round, and he has refused to urge his supporters to vote for Macron in order to keep Le Pen out of the Elysee Palace.

The decisions by those left-leaning voters -- many of whom have expressed a visceral rejection of Macron's pro-business tax cuts and other policies -- could prove crucial.

Looking ahead to parliamentary elections in June, often deemed the "third round" in France's electoral system, Melenchon on Tuesday called for a left-wing alliance that would deny either Macron or Le Pen a majority and potentially set him up as prime minister.

"I will be prime minister, not because Macron or Le Pen want it, but because the French will have elected me," he told BFM television.

Approval slips 

Wednesday's debate, the only one Macron agreed to in this year's race, is a pre-run-off ritual watched by millions and has often proved pivotal in determining the choices of last-minute voters.

Macron's allies have warned him against complacency, not least faced with Le Pen's persistent attacks against the former investment banker as an aloof "president of the rich," out of touch with workaday concerns at a time of rising inflation and insecurity.

An Odoxa poll released Wednesday found that Macron's approval rating as a "good president" had slumped to just 40 percent in mid-April, down six points from March.

That could render the result on Sunday extremely close, even though the survey found that a majority of respondents still find Le Pen's populist, anti-immigration programme racist (56 percent) and divisive for the country (67 percent).

"For the first time, in order to kick out a 'president of the rich,' a large number of French seem ready to elect a president they consider less competent, without sufficient stature to be president," Odoxa's president Gael Sliman wrote.

"This debate will probably be decisive for giving an advantage to one of these two rivals," he said.

Zelensky weighs in 

Macron will likely seek to portray Le Pen as a fringe politician who cannot be trusted on foreign policy -- especially after Russia's invasion of Ukraine, given her past support for President Vladimir Putin.

Ukraine's President Volodymyr Zelensky entered the French debate Wednesday by urging Le Pen to admit "she made a mistake" in her admiration for Vladimir Putin and her refusal to condemn his 2014 annexation of Crimea.

If she did, "our relationship could change," Zelensky told BFM in a video interview, but "obviously I have ties with Emmanuel Macron and I would not like to lose them."

Appealing to French people to vote for Macron, jailed Russian opposition leader Alexei Navalny was blunter.

In Twitter posts, he accused Le Pen of "corruption" and "selling political influence to Putin" over a 2014 loan of nine million euros ($10 million) from a Russian bank he called "Putin's notorious money-laundering outfit".

Macron is also likely to target Le Pen's plans for limiting the economic impact of the Ukraine war for low-income households, and her promise to give "national priority" to French citizens for jobs or welfare benefits.

For her part, the far-right leader will zero in on Macron's proposal to push back the retirement age from 62 currently -- though in recent days he has wavered on whether it should be 65 or 64.