Former prime ministers Francois Fillon and Alain Juppe were going head to head Sunday in a runoff vote for France's center-right presidential nomination, with the winner likely to face a showdown against the resurgent far right in next year's election.

Opinion polls show Fillon, a social conservative with a deep attachment to his Catholic roots, as the clear favorite after stunning his centrist challenger with a massive surge in support just before Les Republicains' first round of voting last Sunday.

Former French prime ministers Francois Fillon, left, and Alain Juppe. AP

Polls indicate both candidates would beat far-right leader Marine Le Pen next spring. They suggest that Juppe would be better placed to rally left-wing voters and would win by a more comfortable margin.

Fillon, 62, a racing car enthusiast who lives in a Loire Valley chateau, promises radical reforms to France's economy, vowing to roll back the state and slash government's bloated costs.

Juppe, 71, a moderate who is currently mayor of Bordeaux, has attacked the "brutality" of Fillon's reform program and says he lacks credibility.

But in a blow to his comeback bid, television viewers found Fillon more convincing in a head-to-head debate Thursday.

"My enemy is the decline of France," Fillon declared Friday night, speaking to supporters in Paris at a final rally before the vote.

President Francois Hollande, whose low popularity ratings mirror the disarray in the ranks of the left, has two weeks to decide whether to run for re-election.