French voters will decide between Francois Fillon and Alain Juppe to be their country's centre-right candidate ahead of next year's elections on Sunday (27 November).
As the second round of voting gets under way, polls are forecasting 62-year-old Fillon will triumph over Juppe in the nomination after securing 44% of the votes in the first round a week ago.
The Republicans' chosen candidate is widely believed to be the next occupant at the Elysee Palace with the ruling party's Francois Hollande of the Socialists in disarray and many wanting to avoid the far-right politics of Marine Le Pen of the National Front.
Former president Nicolas Sarkozy was knocked out of the race in the first round of voting.
Polls opened at 7am GMT and will close at 6pm GMT.
Both Fillon and Juppe propose supply-side economic measures including cuts in public spending of at least €100bn (£85bn, $106bn) and raising the retirement age, although Fillon promises more drastic measures at a faster pace.
In a televised debate on Friday, the BBC reported Fillon said: "I've got a plan for the reforms I want to make in the first three months of the presidential term - and I'm convinced if we don't get these changes implemented in the first three months, the French people will feel disheartened.
"They'll turn away from politics, and then there'll be a greater risk of the extremists winning."
However, Juppe hit back at Fillon, saying "reform should not be a punishment but bring hope".
"The French social model exists, I want to consolidate it, not break it."
Though one of the two men are expected to triumph against Le Pen, that prediction is far from guaranteed with far-right nationalism on the rise and pollsters discredited after incorrectly calling both Britain's referendum on the European Union and the American presidential elections.