His prime minister was among the last to know. That is how secretive, how confined to a small group of advisers President Emmanuel Macron’s shock decision to dissolve Parliament and call French legislative elections was.

Gabriel Attal, 35, was a personal favorite, his wunderkind, when Mr. Macron named him prime minister in January. Yet, just months after entrusting Mr. Attal with the task of revitalizing his government, Mr. Macron snubbed him as he considered one of the most important decisions of his presidency: whether to call an election at the very moment the anti-immigrant National Rally party had surged.

Mr. Macron’s style has always been intensely top-down, but this time he has played with the possibility of ushering in the once unthinkable in the form of a far-right government. The small group making the decision was so insular that even many of his ministers and supporters were left dumbfounded at his readiness to take such a gamble.

A photograph posted by Mr. Macron’s official photographer on Instagram captured the dismay when, on June 9, Mr. Macron told his government of his decision. Mr. Attal, arms crossed, looks blank. Gérald Darmanin, the longtime interior minister who has since announced he will likely leave the government, looks incredulous, his hands clasped in front of his face.

Mr. Macron, defining himself as an “incorrigible optimist,” insists he had to call the election, which would leave him as president but could force him to share power with his sworn opponents for his final three years in office. His favorite word has become the “clarification” that he says only a national vote can deliver. After his party was trounced by Marine Le Pen’s National Rally in European Parliament elections, to have carried on as if nothing had happened would have been to show contempt for democracy, he told journalists.

Still, nothing obliged him to hold a snap election, just weeks before the Paris Olympic Games, that could bring the nationalist right to power.