Sarkozy is returning to the helm of the party he helped to create. His opponents - Bruno Le Maire, 45, and Herve Mariton, 56 - are lacklustre former ministers.
Beneath the surface, though, Sarkozy's return is highly risky.
He is mired in legal woes, notably over the financing of his 2012 presidential campaign. If he wins, Sarkozy will inherit a party that under one of his lieutenants, Jean-Francois Cope, descended into chaos, racking up 74.5 million ($118 million) in debt.
Cope stepped down in June in a scandal involving fake bills for services for the 2012 campaign, leaving the UMP in the hands of three veterans - former Premiers Alain Juppe, Jean-Pierre Raffarin and Francois Fillon.
Juppe and Fillon have Sarkozy most in their sights. The pair, both rugby fanatics, have each declared plans to tackle the party's candidacy in 2017, and their dislike of Sarkozy runs deep. There are signs that a dirty race for the primary lies ahead.
Fillon, according to Le Monde, privately urged Hollande's chief of staff to press ahead with legal charges against Sarkozy. "Hit him quickly, hit him quickly ... if you don't hit him quickly, you'll see him come back, so do it," Fillon reportedly said. Fillon denies the report.
Juppe and Fillon wield clout among UMP centrists who accuse the former President of factionalism and making populist shifts to the right.
A row erupted in the UMP last week after a rally in Bordeaux where Juppe, the popular local mayor, was loudly jeered by Sarkozy supporters. "I don't like one of us being booed ... but I'm not going to be the one who gags UMP members," Sarkozy said, a touch disingenuously. The UMP "is the family of freedom, of freedom of thought, speech and discussion".
Sarkozy further alienated the party's liberals by backing demands to repeal a Socialist law enabling gay marriage. And, in a meeting among diehard supporters on his home ground in Paris, he said he had appointed Rachida Dati as his Justice Minister in 2007 because of her Arab background. "I told myself that Rachida Dati, with her Algerian and Moroccan mother and father, made common sense when it came to talking about crime policy," Sarkozy said, igniting criticism that he had been guilty of either tokenism towards France's immigrant population or borderline racism. "I'm not racist - after all I gave Rachida Dati a job," the campaign group SOS Racism said in an ironic tweet.
According to a survey last week by the pollsters Odaxa, Sarkozy is backed by 63 per cent of UMP members, compared with 73 per cent a month ago.
The big beneficiary of the turmoil among the Socialists and the UMP is the far-right National Front. According to present expressions of voter intent, the former pariah would make it to the runoff in 2017 under France's two-round ballot system.
Nicolas Sarkozy
Age: 59
Who: Centre-right former French President
Family: Sarkozy is married to Carla Bruni-Sarkozy
Political background: Sarkozy was President for five years but lost in 2012. He is seeking to become leader of the opposition UMP by party vote this weekend
What does he want: Sarkozy wants to turn the UMP into a vehicle to re-elect him President in 2017. He also wants to push the new movement towards the mildly Eurosceptic and anti-immigrant right
