François Fillon claims victory on Sunday night in the French conservative party presidential primary. | Eric Feferberg/AFP via Getty Images François Fillon wins French conservative primary Former prime minister becomes favorite to win presidency in spring elections.

PARIS — François Fillon won conservative party Les Républicains’ presidential primary election on Sunday, shaking up France’s political landscape less than six months before the 2017 general election, in which he now becomes the instant favorite.

Fillon, a 62-year-old who first won a seat in parliament in 1981 and was prime minister under former President Nicolas Sarkozy from 2007 to 2012, won 66.5 percent of the vote to his rival Alain Juppé’s 33.5 percent in the runoff of the primary.

His unexpected victory in last Sunday's first round, which he won with 44 percent of the vote, eliminated Sarkozy from the race and left him facing Juppé, who seemed until then the arch-favorite both in the Right’s primary and the presidential race.

Fillon’s victory speech on Sunday night sounded like one candidates tend to make once they are elected to the presidency. “I’m extending my hand to all those who want to help our country," he said. "No one will feel excluded form a society that will be more fair, and more humane.”

Socialist meltdown

The Républicains' candidate immediately became the target for would-be candidates in the Socialist Party's own primary to be held in January. The common theme of those attacks — that Fillon is an arch-conservative intent on destroying France’s welfare state — did little to hide the deep divisions of the ruling party, which seemed to implode on the day the conservatives chose their candidate.

In an interview published Sunday by Journal du Dimanche, Prime Minister Manuel Valls criticized President François Hollande for his leadership style and hinted that he might run in the Socialist primary even if it meant running against his boss, who is yet to announce his decision whether to run or not.

Invited to comment on the Right’s primary on TV Sunday night, former economy minister and would-be presidential candidate, Arnaud Montebourg, briefly denounced Fillon as a representative of the “hard Right,” then spent more time denouncing Hollande and Valls for their economic policies.

That contrasted with the display of unity put on by Les Républicains, which organized a staged handshake between Fillon and Juppé a little before 10 p.m. on Sunday night.

Juppé congratulated Fillon and wished him a “great victory” next year. Fillon saluted Juppé as a “statesman” and insisted that a fresh scratch he had on his nose had not been inflicted by his opponent, but by a photographers' scrum.

Fillon doctrine

Fillon, whose conservative themes and value-based campaign had been denounced by Juppé, indicated that he would try to unite the Right by pouncing hard on unpopular Hollande and what he called his “pathetic” presidency.

His success among conservative voters showed that a large segment of the electorate embraced his plans to shrink the size of France's extensive welfare state through a mix of tax and spending cuts, and by taking on France’s public-sector unions.

His foreign policy proposals, based on an overall alliance with Russia, came under attack as they would signal a radical change with France’s diplomacy in the last 10 years.

On Europe, however, although he has expressed few ideas, he has suggested that he would not stray from the centrist line all French governments have toed in the last two decades.

The slight hope in the Socialist camp, assuming it can one day unite behind a candidate, is that centrist voters might frown at the hard Right platform Fillon has opted for, notably on education or health care.

But it is for the moment hard to see how that would help the divided Left make it past the first round of the presidential election, due on April 23.

All the latest polls showed the second round as a face-off between the mainstream conservative candidate — whoever he may be — and far-right National Front leader Marine Le Pen.

That poll advantage will only increase in the next weeks as Fillon will benefit from the strong momentum that his unexpected, and unexpectedly strong, primary victory will give to his candidacy.

This article was updated with more results.