“Installing a banker while our country is suffering from the domination of finance is not a good sign,” said Jean-Marc Germain, a Socialist member of Parliament.

It did not help that Mr. Macron made a major faux pas right out of the gate, in a radio interview, referring to the plight of “illiterate” workers at a factory that was closing in northern France, because they would have few other options. The remark, though perhaps well intentioned, deepened an impression that he was out of touch with the Socialist base.

The French news media was quick with labels, calling Mr. Macron “the Right Brain of Hollande” — a reference to his right-leaning economic influence on the president. “Les Guignols de l’info,” a satirical political puppet show, caricatured Mr. Macron as a crying baby, a swipe at his youth and perceived political inexperience.

But Mr. Macron’s supporters say he is no callow upstart.

“He has an amazing expertise, a political mind, and is modern and liberal in his thinking,” said Jacques Attali, an influential French economist and an adviser to several presidents. “He also wants to fight inequality, and he cares about the impact on civil society.”

Mr. Macron met Mr. Hollande in 2007 during a party at Mr. Attali‘s home, and the two men hit it off. At the time, Mr. Macron had just done a surprising about-face from an early career in academic philosophy, working with the late Paul Ricoeur, an eminent French philosopher who focused on human consciousness. Mr. Macron obtained a master’s degree after partly focusing his studies on Machiavelli, which, he said with a smile, was good background for navigating the power politics of Paris.

He entered the École Nationale d’Administration, the training ground for France’s governmental elite, pausing in 2007 to marry his former high school French teacher, Brigitte Trogneux, who is 20 years his senior. In 2008, he was snapped up by Rothschild, working mainly in Paris, a job that made him a millionaire.

When Mr. Hollande ran successfully for the presidency in 2012, he campaigned on a classic leftist platform that promised greater security for the middle class and higher taxes on the wealthy. But behind the scenes, he called on Mr. Macron as an informal adviser to assure the business community that he was also open to reforms that would help companies create jobs and lift France from moribund growth.