PARIS (Reuters) - The campaign of French presidential hopeful Manuel Valls is losing steam as the Socialists enter a crucial week in picking their candidate, polls show, a scenario that could help Emmanuel Macron.

French politician and former Prime Minister Manuel Valls, candidate for the left's presidential primaries, attends a meeting with supporters in Lievin, France January 8, 2017. REUTERS/Pascal Rossignol

The Socialists, who have held the presidency under Francois Hollande since 2012, are not expected to come close to retaining power in the spring election.

Most polls see a runoff on May 7 between conservative Francois Fillon and far-right leader Marine Le Pen, with former prime minister Fillon winning by a comfortable margin.

But polls have also shown Macron, a popular former economy minister, cementing his position as the “third man” who might upset predictions.

The Socialists’ choice of candidate in primary elections later this month could prove critical for any of the front-runners.

According to weekend opinion polls, Valls’ rivals former ministers Arnaud Montebourg and Benoit Hamon are hot on the heels of the former prime minister - which could hurt Le Pen, who leads the anti-immigrant and euro-hostile National Front (FN).

Valls, a pro-business Socialist who has pushed unpopular free-market reforms through parliament, has long been seen as most able to attract centrist voters.

The two weekend surveys showed for the first time anti-austerity left-winger Montebourg winning his party’s ticket.

This could give centrist Macron, who has stayed outside the primaries to run as an independent, a boost by pushing moderate Socialists towards the former investment banker.

“Valls’s defeat may prompt parts of Francois Hollande’s 2012 voters to go to Macron,” Frederic Dabi from pollster Ifop told Reuters, referring to the last presidential election.

Dabi said three prime-time TV debates between the left’s seven contestants - starting on Thursday - will be key.

“The ‘anybody but Valls’ factor may end up framing the debates,” he said.

A survey last week showed Macron’s only chance of qualifying for the runoff on May 7 was if Montebourg was the Socialist candidate.

In this scenario, Macron would beat Le Pen into third place in a first round in April to take on Fillon in the knockout.

An Ifop poll in Journal du Dimanche showed Montebourg winning 52 percent of the Jan. 29 second-round vote in the Socialist primaries - the first time he has scored ahead of Valls in a major poll.

A poll by Kantar Sofres One point in Le Figaro put Montebourg on 53 percent in the second round.

Both polls also showed a narrowing first-round lead for Valls over Montebourg and Hamon.