French President Emmanuel Macron's party is hoping to secure half the seats in the Senate elections today,
His centrist Republic on the Move! party, created just last year, won a large majority in the lower house of parliament in June elections, but is unlikely to do the same in the Senate.
Polls suggest the conservative Republicans party will consolidate its dominance of the chamber's 348 seats instead.
Mr Macron's party is likely to seek alliances in the Senate with other centrists and moderate Republicans and Socialists to approve his business-friendly economic reforms.
The senators are not chosen by the public but by some 75,000 elected officials - mayors, legislators, regional and local councillors - casting ballots in town halls across the country.
Results are expected tonight. Nearly 2,000 candidates are running for 171 Senate seats.
It is the first time Mr Macron's party is competing in Senate elections since he created it to shake up French politics and attract voters tired of the status quo. The party is hoping to win 50 seats.
The election also comes as Macron's popularity is on the wane, just four months into his presidency.
Tens of thousands of people massed in Paris yesterday to protest changes to labour law that they fear are dismantling the French way of life - and more protests and strikes are ahead.
Truckers plan blockades of streets and fuel blockades on Monday.
Mr Macron insists the changes, which reduce union powers and hand companies more freedom to lay off workers, are needed to create jobs and compete globally.