PARIS — A day after mainstream parties were dealt a heavy defeat in the French presidential election, the far-right leader Marine Le Pen, one of the two candidates to advance to a runoff, condemned the parties’ calls to unite against her and support her rival, the independent centrist Emmanuel Macron.

Ms. Le Pen’s statement on Monday denouncing “the old and completely rotten Republican Front” — the coalition of mainstream parties allied against her — sums up her challenge in the May 7 runoff. So far, not a single rival party has called for its voters to support Ms. Le Pen. And she has no plausible major reservoir of votes to add to the 21.3 percent she received in the first round of voting, though she is expected to gain some voters from the defeated center-right candidate François Fillon.

Perhaps in an effort to broaden her appeal to voters from outside the far-right National Front’s traditional constituencies, Ms. Le Pen announced on Twitter on Monday that she was temporarily stepping down as the party’s leader so she could run as a candidate for “all the French.”

“Tonight, I am not the president of the National Front, I am the presidential candidate, the one who wants to gather all the French around a project of hope, of prosperity, of security,” she said in an interview on French television.