PARIS — In the climax of France’s presidential campaign, the centrist president, Emmanuel Macron, and the far-right contender, Marine Le Pen, will meet Wednesday evening in a one-on-one television debate that could prove decisive before Sunday’s runoff vote.

Both candidates have carefully prepared for the highly scrutinized debate, expected to last more than two hours.

Mr. Macron, a pro-European centrist, emerged ahead from the April 10 first round and is leading in opinion polls. Ms. Le Pen, an anti-immigration nationalist who has gained ground this year by tapping anger over inflation, has significantly narrowed the gap in public support compared to the last presidential election five years ago, when she lost with 34 percent of the vote to Mr. Macron’s 66 percent.

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 at some points. 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 criticized for being insufficiently prepared.

Meanwhile, the then-39-year-old Mr. Macron, despite his little political experience, seemed comfortable speaking about all kinds of issues and able to go deep into details in what appeared as proof of seriousness.

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

This time, 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 don’t know whether they will even go to the polls.

Ms. Le Pen is expected to appeal to those who have anti-Macron feelings, criticize his record, and present her populist platform as an alternative. She also aims at demonstrating that she has the stature of a potential president, and at promoting what she says are realistic proposals. Among them are banning Muslim headscarves and scaling back relations with the European Union and NATO.

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 that his pro-business stance shouldn’t 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.

It may be an uphill battle.

“This is a campaign of profound dissatisfaction for many French people,” the managing director of polling firm IPSOS, Brice Teinturier, said Wednesday on LCI television. Instead of seeing this election as a chance to dream of a better future, he said many French voters are feeling “weariness, worry, sometimes frustration or even anger.”

Some voters in Paris expressed hope that the debate would call attention to climate concerns, or education, or the importance of a strong Europe, or see Ms. Le Pen take “revenge” on Mr. Macron after their 2017 duel.

By SYLVIE CORBET. Catherine Gaschka and Oleg Cetinic contributed to this report.