"The French never vote like the Americans," a politician told a rally for presidential favourite Alain Juppe on Wednesday as France wondered if it would be the next country to prove the opinion polls wrong.

Juppe, mayor of the southwest city of Bordeaux and a former prime minister, has topped the polls for months. Surveys predict he will win both a primary later in November to be the centre-right's candidate and the presidential election in six months.

But after pollsters failed to predict US voters would make Donald Trump their country's 45th president, like their British colleagues who got the United Kingdom's vote to leave the EU wrong, France has started bracing for a possible surprise next year.

"Juppe is ahead in polls and he will also win. The French are not like the Americans, we're not crazy," Juppe supporter Mbacoye Balde, 35, told Reuters at the Bordeaux rally.

Nearby, another Juppe supporter carried a banner reading "Ne vous Trumpez pas" a French play on words with Trump's name that meant "Don't make a mistake."

"We've always been cautious about polls, we've always said one should not get carried away and it was not in the bag," Virginie Calmels, Juppe's deputy in Bordeaux and one of his campaign spokespersons, said of his strong ratings.

But she added: "It's not quite the same in France and in the United States." Unlike in the United States for the Trump vote, pollsters in France have recently tended to overestimate support for the anti-immigration National Front (FN), she said.

The far-right party won no region in last year's local elections, despite forecasts it would get at least one.

But its leader Marine Le Pen is another strong candidate in the presidential election and is expected to be one of the two contestants to make it into the runoff round - possibly against Juppe himself.

Juppe told the rally that, if elected, he would "obviously be available for dialogue with President Trump" but spent much of his speech warning against populism, saying inclusiveness and hope were the answer.

Juppe is campaigning on a moderate platform more to the centre than the law-and-order strategy of main rival Nicolas Sarkozy, who was France's president in 2007-2012 and has courted controversy with his hard line on immigration and Islam.

Without mentioning Sarkozy by name, Juppe said: "Beware the false answers and bad solutions ... I say 'no' to divisiveness, 'no' to demagoguery that pit the French against one another."

"I want the optimistic France to lift up the sad France," he said, just days ahead of the two-round primary on November 20 and 27 where he will compete against Sarkozy and five other candidates for the centre-right's nomination.