Jean-François Copé, the party's firebrand secretary-general, declared to cheering supporters after voting closed late Sunday that he was the victor, but ex-prime minister François Fillon announced just minutes later that he had won.
Copé repeated his claim on breakfast television on Monday, as Fillon supporters took to the airwaves to insist their man had triumphed in the vote that came six months after Sarkozy lost the presidential election to the Socialist François Hollande.
Both camps claimed there were irregularities – and cheating – in voting in several areas, and the party's electoral commission was Monday recounting ballots and aiming to declare the official winner later in the day.
The public slanging match, reminiscent of the bitter infighting that for years dogged the Socialist party, reached such a pitch that #UMP became the top trending hashtag on the French edition of Twitter, where the candidates were the targets of a deluge of mockery.
UMP party heavyweight Alain Juppé, who was Sarkozy's foreign minister, pleaded with the pair to put a stop to their supporters' "invectives" and warned that "the very existence of the UMP is in question".
"The movement has emerged divided and thus weakened by this excessive confrontation. Throughout the campaign, it has been less a question of the future of the UMP and more about the two candidates' obsession with 2017," he wrote on his blog.
That is the date of the next presidential election.
Most UMP supporters want Sarkozy to stand for presidency
But whoever emerges as the new leader is not certain to be the party's candidate then as Sarkozy – whom polls say most UMP supporters want to have another tilt at the presidency – has not ruled out a return to politics.
Both Fillon, 58, and Copé, 48, are advocates of free market policies and economic reform. But they differ on social issues, with Cope sharing Sarkozy's tough-talking approach to immigration and Islam.
Aides to Copé, who accused Fillon supporters of "ballot stuffing" and "major fraud", said he had won 1,000 more votes than his rival in a poll in which more than half of the UMP's 300,000 members had cast their ballots.
Fillon said he was 224 votes ahead.
Fillon appeared more conciliatory on Monday morning. He still insisted he was ahead but added in a statement that "only the official and definitive figures will enable us to resolve the situation".
Whoever emerges as the new UMP leader will be taking over a party well-placed to capitalise on Hollande's slump in popularity and the economic gloom engulfing France.
But he may also face a difficult task in uniting the party after a bitter battle that delighted the UMP's rivals.
"It is obvious that whoever is elected president of the UMP will have no legitimacy whatsoever given that he will be in charge of a party broken in two," said Florian Philipott, deputy leader of the far-right National Front.
Fillon, who was prime minister for five years under Sarkozy, went into the vote as the marginal favourite, hoping to sell himself as a unity candidate capable of attracting centrist voters.
He accused his rival of opportunism, while seeking to portray himself as an experienced statesman – a stance that prompted Copé to dismiss him as the "Hollande of the right," in a reference to the president's perceived lack of charisma and reputation for dithering.
Copé has taken up where Sarkozy left off, unabashed in his bid to woo voters from the far-right National Front, whose historically strong score at this year's presidential election split the right-wing vote and torpedoed Sarkozy's re-election bid.
Copé last month published "A Manifesto for an Uninhibited Right", in which he lambasted a culture of "anti-white racism" amongst immigrant communities in impoverished urban areas.