PARIS — Nicolas Sarkozy's hopes of running for a new French presidential term in 2017 faced a crucial first test on Saturday as members of his crisis-ridden conservative UMP party voted for a new leader.

The online vote, which the former head of state is expected to win, was slowed on Friday evening by a cyber attack on the election web site, but voting continued on Saturday and a result was expected in the evening.

Sarkozy won the French presidency in 2007 but has watched impatiently from the sidelines since losing to Socialist Francois Hollande in 2012.

He is favourite to be named UMP chairman but knows only a convincing win will deter party rivals from emerging later to challenge him for the presidential ticket.

"A result below 70 percent would be in fact a defeat for him, because he is so much the charismatic savior, that's how he wants himself to be seen," Dominique Moisi of the French Institute for International Relations (IFRI) told Reuters TV.

"Less than that score would not do, and would be used by his opponents as absolute proof that his return is a failure."

With Hollande's ratings sent to record lows by tax rises and a failure to tackle unemployment, the opposition conservatives should be flying high. This week's economic data showed the country's jobless rate at the latest in a series of record highs and consumer spending in the doldrums.

But the UMP has long been riven by leadership squabbles and is mired in a legal inquiry into alleged funding irregularities, while some of its voters have been lured away by a resurgent far-right National Front led by Marine Le Pen.

A political brawler who polarizes French opinion and makes no secret of his dim view of Hollande, Sarkozy stormed back in September with a right-wing platform aimed at winning voters from Le Pen and re-founding his 12-year-old party.

Sarkozy says he is the innocent victim of a politically-motivated plot, but concerns about legal cases have weighed on his ratings.

A survey last week by Odoxa pollsters showed just 63 percent of UMP backers want him back at the helm, 10 points down in the month. His main challenger for the chairmanship, Bruno Le Maire, may yet get enough votes to encourage other party heavyweights to challenge Sarkozy for the UMP presidential ticket.

Le Pen is herself due to be confirmed unopposed as National Front leader at a party congress vote in Lyon on Sunday.

A UMP statement said more than 100,000 of the party's 268,341 members had cast their votes by midday on Saturday.

Late on Friday it said the cyber attack had been "one of the risks anticipated" and had only succeeded in slowing the voting process.

Acting UMP head Luc Chatel said the party had lodged a complaint with Paris police and called the attack "organized", but he said the turnout had been strong and that the incident would not put the validity of the vote in question. (Reporting by Gregory Blachier and Andrew Callus, editing by Rosalind Russell)