Mr. Philippe, the mayor of the northern port city of Le Havre, is an ally of Alain Juppé, a former prime minister who ran in the center-right presidential primary.
Turnout will be important: The latest polls say about a quarter of France’s voters may abstain because they do not support either candidate.
A trove of internal campaign documents has been published online, the campaign staff for the French presidential candidate said.
Supporters of the National Front contender, who used words similar to those of her conservative ex-opponent François Fillon, said they were a “small loan.”
Ms. Le Pen said she had reached an agreement with Nicolas Dupont-Aignan, a right-wing politician who shares her distrust of the European Union.