PARIS — Not since World War II has the anti-immigrant far right been closer to gaining power in France. With her second-place finish on Sunday in the first round of the presidential election, Marine Le Pen has dragged her National Front party from the dark fringes of its first 40 years.

But that remarkable accomplishment is so alarming to so many in France that as soon as the preliminary results were announced at 8:01 p.m., virtually all of her major opponents in the 11-person race called for her defeat in the second-round runoff on May 7. They implored their supporters to vote for the candidate projected to come out on top on Sunday, the centrist, pro-European Union former economy minister Emmanuel Macron, a political novice and outsider.

The first-round showing by Mr. Macron and Ms. Le Pen represented an earthquake, as they effectively broke the French political establishment. On the right and the left, the two parties that have governed France for more than 50 years suffered a severe defeat. They have been pushed aside in a wave of popular anger over the country’s stagnant economy and shaky security.

The rapid-fire endorsements of Mr. Macron, coming from across the political spectrum, represented a dynamic that has always prevailed in France when the National Front approaches executive power — the cross-party, anti-far right alliance the French call the “Republican Front.” The question now is whether that front can hold this time, as well.