Ahead of the second round of the US-style primary on Sunday, with the winner tipped to become France's next president, frontrunner Francois Fillon and his rival Alain Juppe traded barbs over abortion, gay marriage and the pontiff's teachings.
Fillon, a devout Catholic father of five, surged from behind in the first round of the primary last Sunday to take the top spot on a platform of radical economic reforms and social conservatism.
His supporters include some of the tens of thousands of people who demonstrated against a gay marriage bill in 2013.
Fillon has ruled out revoking the law but pledged to amend it to prevent gay couples from being able to adopt children.
The 62-year-old solicitor's son, who voted in 1982 against decriminalising homosexuality, has also said he personally disapproves of abortion but supports a landmark 1975 law making terminations legal.
Juppe, a twice-married 71-year-old "agnostic Catholic" who like Fillon is a former prime minister, on Tuesday blasted his rival's "extremely traditionalist, not to say slightly retrograde view on the role of women, family and marriage".
Fillon, he said, should "clarify his position" on abortion -- drawing a furious reaction from his opponent, who reiterated that he supported women's right to choose and accused his rival of hitting "below the belt".
Desperate to make up ground on Fillon, who led the first round by 18 percentage points, Juppe also invoked Pope Francis -- a rare campaign reference in secular France.
The Bordeaux mayor suggested his positions were closer than his rival's to those of the modernising pontiff.
"You have to... keep in step with the times," he said.
Fillon hit back at what he called the "ultra-grotesque" stereotyping of him as a "reactionary from the Middle Ages".
"I make no apologies for having values," he told Europe 1 radio on Wednesday, declaring that on "most" of the subjects raised by Juppe "Pope Francis says the same thing as I do".
In a primary campaign that has focused mainly on France's ailing economy, immigration and security after a wave of jihadist attacks, the sudden emphasis on societal issues reflects the importance of the Catholic vote.
While practising Catholics make up only about 10 percent of the population, "they are far more likely to vote than average, which gives them more clout," Jerome Fourquet of Ifop pollsters told AFP, estimating that they make up 15 percent of the national electorate.
Aware that their support could be key to winning the nomination, Juppe has balked at extending rights for gay couples beyond marriage to areas such as fertility treatment.
Like Fillon, he opposes giving lesbians access to IVF and supports a continued ban on surrogacy -- one of the paths to parenthood taken by gay men.
At the prestigious Stanislas Catholic school in central Paris, parents and teachers were divided on the candidates' merits.
"I voted Fillon because of his positions on family and homosexuality," said Camille Roullier, a 38-year-old mother.
Mathilde, a 31-year-old teacher who declined to give her surname, said she too supported the man from the north-central city of Le Mans because of his position on gay adoptions.
But another of the teachers, Amedeo, 59, endorsed Juppe.
"I'm for a 'live and let live' approach," he said.
In an editorial, the leftist daily Liberation portrayed Fillon as the standard bearer of an "aggressive, activist, political Catholicism that mirrors political Islam".
But the managing editor of La Vie, a Christian weekly, rejected the idea.
"There is no political Catholicism in France and there never will be," Jean-Pierre Denis said, noting that a Christian Democrat candidate knocked out of the primary in the first round had mustered only 1.5 percent of the vote.
by AFP's Beatrice Le Bohec and Benoît Fauchet