"Aren't you ashamed to repeat claims by a man who has served time in jail and who has been found guilty several times of defamation and is a liar?" Sarkozy exclaimed when moderator David Pujadas asked him to comment on Franco-Lebanese businessman Ziad Takieddine's claim that he transported three suitcases filled with cash from Libya to France in 2007.
Thursday evening's generally restrained debate flared up when Sarkozy was quizzed about the allegations that he received millions in funding from the former Libyan leader Moamer Kadhafi.
He slammed state-owned TV station France 2 for allowing such a "disgraceful" question to be posed.
In two interviews with police this week, on Tuesday and Thursday, Takieddine, a middleman in huge arms and oil contract in the Middle East, has repeated his claim that he carried suitcases stuffed with 1.5 million to two million euros in 200-euro and 500-euro notes.
Several former Libyan officials have confirmed the accusation that Kadhafi funded Sarkozuy's campaign and notebooks left by former oil minister Choukri Ghanem, who died in 2012, mention it.
Seven candidates fight for right's nomination
Seven candidates are standing in primaries that take place next Sunday, with a deciding round the Sunday after.
Six - Sarkozy, former prime ministers Alain Juppé and François Fillon, Bruno Le Maire, Jean-François Copé and the only woman, Nathalie Kosciusko-Morizet - are members of the Republicans party.
The seventh, Jean-Frédéric Poisson, heads the small Christian Democrat Party.
Opinion polls put Juppé in the lead with Sarkozy and Fillon now vying for the second place that would mean going into the second round.
Trump win changes Europe's role
The surprise result of the US presidential election was on the candidates' minds, with Kosciusko-Morizet saying she didn't want France to "wake up with a hangover, like the Americans, in six months' time".
has created "a fantastic opportunity for France and Europe to reestablish a leadership role" on issues including security in the Mediterranean and the reform of the UN Security Council, Sarkozy argued.
"The next five years will mark the return of France and Europe to the international scene. America won't be there to put us in the shade," he said.
Juppé said the Trump era heralded a triple "shock" - in trade, defence and the environment.
A return to protectionism would be "a tremendous regression", he claimed, while warning Europe against being "naive" in its dealings with the United States.
Both Sarkozy and Fillon welcomed the prospect of a rapprochement between Washington and Moscow under Trump.
"The good news" of the US upset, Sarkozy said, was that it spelt an end to the "totally counter-productive climate of Cold War between the United States and Russia".