CREPY-EN-VALOIS, France (Reuters) - Denise Rollet says she faces no good options in France's run-off legislative vote in the coming days.

The 80-year-old former teacher from Crepy-en-Valois, a small, middle-class town northeast of Paris, must now pick a candidate from two parties she would never vote for: Marine Le Pen's far-right National Rally (RN), or and a Socialist standing as part of a hastily assembled leftist alliance.

"On Sunday, things get serious, and frankly, I have no idea who I'll vote for," she said.

Across France, millions of moderate voters are facing what they regard as grim choices ahead of Sunday's crucial legislative run-off vote that could hand power to the RN.

The party has sought to rebrand itself as a mainstream right-wing party focused on immigration and pocketbook issues, but remains a pariah for many in France due to its past association with racism and antisemitism.

The RN scored historic gains in Sunday's first-round vote. However, it remains unclear whether it will win the majority it says it needs to govern France in an awkward "cohabitation" with centrist President Emmanuel Macron.

Sunday's results sparked a bout of furious horsetrading. Third-placed centrist and leftist candidates struck deals to sit out the run-off to prevent the anti-RN vote from splitting in a long-running political ritual known as the "republican front".

The poor showing of Macron's alliance, which came third in Sunday's vote, underlines the collapse of the traditional mainstream during his seven years in power, pushing voters to more hardline parties on the left and right.

For some heading to the ballot box on Sunday, that means few palatable choices.

Charles-Edouard Parent, a retiree in Crepy-en-Valois, said he and lots of others he knew would be casting a blank vote "so as not to have to choose between these two extremes".

Crepy-en-Valois is part of the fifth constituency of the Oise, a mostly conservative district that has been held by the centre-right Republicans (LR) since 1993.

The one-time party of former French President Jacques Chirac, the LR has been a victim of the hollowing out of the political centre. The party split before the first-round round vote with a small number of its lawmakers decamping to the RN. Many LR voters in Crepy-en-Valois followed suit.

WILL THE 'REPUBLICAN FRONT' HOLD?

Frédéric-Pierre Vos, the RN candidate for the Oise's fifth constituency, easily won the district with more than 42% of the vote.

A Le Pen confidant and RN party lawyer who helped engineer the 2015 expulsion of her father, Jean-Marie Le Pen, from the party for antisemitic remarks, Vos acknowledged he had been parachuted into a constituency with which he had few links.

Despite calls by Macron and other centrist and leftist leaders for the French to vote for non-RN candidates, Vos was confident of a run-off victory.

He believed his chances were bolstered by the fact he was facing a leftist opponent after Pierre Vatin, the LR incumbent who had represented the district since 2017, failed to make the second round.

Sudhir Hazareesingh, a French politics expert at Oxford University, said Vos' instinct was likely correct.

Leftist electorates "are generally more willing to vote for a centrist to keep out the extreme right", Hazareesingh said, but "centre and right-wing voters are more reluctant to vote for a candidate of the left against the RN in the second round".

Former businessman Pascal Odent, 71, voted for the RN as a bulwark against the "incoherent" hard-left. In an interview in the landscaped gardens of his handsome stone home, he said it was also time to give the far right a chance.

"I'm not looking for grand economic ideas," he said. "We just want our lives to remain pleasant, and that our standard of living is maintained, that it is agreeable."

Bertrand Brassens, a former civil servant and Socialist member of the leftist alliance, came second in Sunday's vote, 20 points behind Vos.

Brassens said his chances of winning a second-round victory, which would likely be narrow, depended on winning over Macron supporters and the small remaining cohort of LR voters who were no fans of the left but even less in tune with the RN.

"For many people, I'm a socialist," he said. "But I'm a republican socialist, in every sense of the word."

He hoped the "republican front" would hold, but was uncertain.

"If it works 100%, I'm elected," he said. "If it holds 50%, I lose."

(Reporting by Sybille de La Hamaide; Writing by Gabriel Stargardter; Editing by Alex Richardson)

By Sybille de La Hamaide