PARIS — The man Marine Le Pen chose to lead her far-right party while she ran for the presidency has been forced to step down because he praised a Holocaust denier and expressed doubt that the Nazis used poison gas to murder Jews.

In a 2000 interview, the man she named as the National Front’s interim leader, Jean-François Jalkh, said Zyklon B, the gas used to kill millions of Jews during the Holocaust, would have been “impossible” to use in “mass exterminations.”

Ms. Le Pen called the outrage directed at Mr. Jalkh a “defamation” on Friday morning.

The negative news, however, was somewhat offset by day’s end, when Ms. Le Pen got an endorsement from a defeated right-wing presidential candidate, Nicolas Dupont-Aignan, who won less than 5 percent of the vote nationwide.

“Marine Le Pen is not on the far right to me,” he said.

Ms. Le Pen is one of two candidates to advance to the second round of the presidential race, and announced on Monday that she would temporarily step down as National Front leader to focus on defeating the independent centrist Emmanuel Macron, considered to be the favorite in the election on May 7.

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Ms. Le Pen herself prompted an outcry earlier in the presidential campaign when she said that France had not been responsible for the roundup of Parisian Jews during World War II in an event known as the Vel’ d’Hiv.

But her efforts to buff her party’s image were complicated this week by a furor over comments that Mr. Jalkh made in 2000. During an interview, a doctoral student, Magali Boumaza, had asked him about the gas chambers. Mr. Jalkh replied that he was not a “negationist” but that he had read the works of a “trustworthy revisionist”: Robert Faurisson, a former professor of history at the University of Lyon who has been convicted of inciting hatred and racial discrimination and who has often been cited by the more extreme far-right elements.

Mr. Jalkh, who once took part in a ceremony commemorating the death of Philippe Pétain, the leader of the Vichy government that collaborated with Nazi Germany, said in the interview that he had been “surprised” by the “rigor “ and “conscientiousness” of Mr. Faurisson’s research.