In the climax of France’s presidential campaign, centrist President Emmanuel Macron and far-right contender Marine Le Pen are poised to go head-to-head in a television debate.

Both candidates have carefully prepared for the highly scrutinised debate that is expected to last more than two hours and could prove decisive before Sunday’s runoff vote.

Mr Macron emerged ahead from the April 10 first round of voting and is leading in opinion polls with a margin varying between three and 13 percentage points.

But Ms Le Pen has significantly narrowed the gap compared to the last presidential election five years ago, when she lost with 34 per cent of the vote to Mr Macron’s 66 per cent.

In 2017, a similar debate struck a decisive blow to her campaign.

Ms Le Pen had looked hesitant, seeking answers from notes piled up in front of her, and appeared to lose her composure.

She also made basic mistakes on several economic topics, which Mr Macron immediately pounced on.

That proved disastrous for her image. Even in her own camp, she was criticised for being insufficiently prepared.

Meanwhile, the then 39-year-old Macron, despite his limited political experience, seemed comfortable speaking about all kinds of issues and able to go deep into details.

Ms Le Pen recently called the 2017 presidential debate the “biggest failure” of her political career.

For Wednesday’s debate – scheduled to start at 5am Thursday AEST – she has pledged to be better prepared, working “at home” with her closest advisers.

Both candidates need to broaden support before Sunday’s vote.

Many French, especially on the left, say they still do not know whether they will even go to the polls.

Ms Le Pen is expected to appeal to those who have anti-Macron feelings, criticise his record and present her nationalist, anti-immigration stance as an alternative.

She also aims to demonstrate she has the stature of a potential president, and will promote what she says are realistic proposals.

Mr Macron, meanwhile, will advocate his pro-European views as the way to make France stronger in the world.

He will seek to convince leftist voters his pro-business stance should not deter them from choosing him.

In recent days, he acknowledged some would back him only to counter the far-right candidate.

“I want to convince women and men with diverse political points of view,” Mr Macron said.

-AP