The UMP, the political heir to the party founded by Charles de Gaulle after World War II, is hoping to move on from bitter political infighting and financial scandals that have pushed it to the verge of collapse.
And Sarkozy, who broke the worst-kept secret in French politics when he announced his political comeback in September, has vowed he is the man to rebuild "the party of hope".
The electronic vote kicked off on Friday night and was hit by a cyber-attack which slowed voting, prompting the party to file a complaint with police. One candidate, Herve Mariton, warned thousands had been unable to vote.
The beleaguered party, still haunted by a 2012 leadership vote which degenerated into accusations of fraud and ballot stuffing, said such an incident was "expected" and had been swiftly dealt with.
Pollsters expect the 59-year-old Sarkozy to sail through the election despite his much-heralded return to politics largely seen as having fallen flat, with his tangle of legal woes continuing to dog him.
The vote pits Sarkozy against main rival Bruno Le Maire -- a former minister and senior party figure -- and lawmaker Mariton.
However a victory is not enough.
Analysts say that anything less than 75 percent of the vote would be a blow to Sarkozy's endgame, if yet unstated: to win back the keys to the Elysée palace which he lost in a humiliating poll defeat to Francois Hollande in 2012.
The energetic former leader has criss-crossed the country to drum up support for his candidacy at the head of the party all the while slamming the "mediocrity" of Hollande's deeply unpopular Socialist government.
"Staying in the background when everything is going so badly would be cowardice, the opposite of my idea of political commitment," Sarkozy told a political meeting on the eve of the vote.
Voting was to wrap up at 7pm with results expected shortly after.
The far-right National Front -- flying high after making gains in local and European elections this year -- also held its party congress on Saturday.
The party, whose founder and former leader Jean-Marie Le Pen has been repeatedly convicted for inciting racism and holocaust denial, welcomed a third generation of the political family into its leadership.
Marion Marechal-Le Pen, the blonde, 24-year-old niece of current leader Marine Le Pen, was elected to the party's executive committee.
For Marine Le Pen, the messy state of mainstream parties is the gift that keeps on giving. A recent poll showed she would win the first round of voting in 2017 with 30 percent.
With Hollande taking a whipping in opinion polls, experts say the 2017 election could end up a race between the UMP candidate and Le Pen.
For Sarkozy's race to the presidency, the real battle of opposition titans will come at party primaries due in 2016.
Chief among these is his former colleague turned arch-foe Alain Juppe, a popular politician and one-time prime minister who served as defence and then foreign minister under Sarkozy.
Sarkozy has as many devotees as rivals in the UMP, which is currently run by a trio of former prime ministers appointed after former leader Jean-Francois Cope was forced to resign in May over a campaign funding scandal linked to Sarkozy's last election bid.
Deeply unpopular at the time of his 2012 election defeat and known as the "bling-bling" president for his flashy style, Sarkozy is hoping to capitalize on the fact that Hollande is now even more disliked by French voters than he was.
Record unemployment, an economy in the doldrums and a string of scandals in his government have seen Hollande's approval ratings sink to record lows.
Andrew Knapp, an expert in French politics at Britain's University of Reading said Sarkozy's bid for the presidency was "partly an act of revenge for a defeat which he has never fully accepted".