Declaring himself an “incorrigible optimist,” President Emmanuel Macron of France appealed on Wednesday to all French people “who reject the extremes” to vote for centrist parties in snap elections and so save the Republic from the bigotry of the far right and the antisemitism of the extreme left.

At a two-hour news conference, a third of it consisting of a speech by the president, Mr. Macron painted a somber picture of economic chaos, lawlessness and a society where there would be “true French citizens and lesser ones” if Marine Le Pen’s National Rally came to power.

“I do not intend to hand the keys of power to the extreme right in 2027,” Mr. Macron, who is term limited, vowed, alluding to the next presidential election.

Yet by dissolving the National Assembly and calling parliamentary elections starting 18 days from now, Mr. Macron has opened the possibility that he may have to hand over some of those keys in 2024. His gamble that the National Rally, which won more than double the vote of Mr. Macron’s centrist alliance in European Parliament elections on Sunday, will not repeat that performance in a domestic vote is a high-risk one.

If the National Rally emerges as by far the largest party in the elections on June 30 and July 7, as polls currently indicate it will, Mr. Macron may be obliged to name a prime minister from Ms. Le Pen’s party, probably its president, Jordan Bardella, 28.

Asked why he had rolled the dice, Mr. Macron said it was essential to have a “clarification,” a word he returned to multiple times. It would have shown disrespect for the will of the people, he insisted, if he had ignored the fact that “50 percent of French people voted for the extremes” in the European election — a reference to votes for far-right and far-left parties.