Both candidates are particularly keen to woo the electorate of hard-left candidate Jean-Luc Melenchon, who finished a strong third in the first round.
Incumbent leader Emmanuel Macron and far-right challenger Marine Le Pen will go head-to-head in a crunch TV debate, seeking to sway undecided voters.
The live televised debate is scheduled from 1900 GMT on Wednesday, four days ahead of the presidential election's decisive second round.
Macron holds a solid poll lead, but his political allies have warned against any complacency in the prime-time duel — their only direct clash — which will be watched by millions.
Some polls are predicting a lead of around 10 points for Macron over Le Pen in the run-off, a repeat of the 2017 election. But undecided voters and abstentions could yet swing the figures.
Opinion polls currently put Macron at 53 to 56 percent in the run-off against 44 to 47 percent for Le Pen — a much tighter finish than five years ago, when Macron carried the vote with 66 percent.
For Marine Le Pen, the duel represents a final chance to win back ground in the polls and convince France she has moderated her anti-immigration party into a mainstream force.
Macron will likely seek to portray Le Pen as a dangerous extremist who cannot be trusted on foreign policy — given her past comments in support of President Vladimir Putin.
READ MORE: EU’s anti-fraud body accuses Le Pen of embezzling French funds
Le Pen has cleared her diary to concentrate on preparing for the debate, hoping to avoid any repeat of the fiasco five years ago, when her ill-prepared performance contributed to her defeat.
This year's vote will mark the closest the far right has come to taking the Elysee presidential palace.
Marine Le Pen's father Jean-Marie was crushed by Jacques Chirac in the 2002 run-off election, and she was defeated by Macron in 2017.
The debate has been a political tradition in France since 1974, when Socialist Francois Mitterrand took on centrist Valery Giscard d'Estaing.
But it did not take place in 2002, when Chirac said debate was impossible with "intolerance and hatred" after Jean-Marie Le Pen stunned France by making the run-off.
READ MORE: French prime minister to resign if Macron is re-elected, eyes 'new impetus'