LONDON — Can a graduate of France’s elite schools, a onetime investment banker and the former economy minister of an unpopular Socialist president prevail at a time of French disgust with politics-as-usual?

I ask the question because the greatest danger to Emmanuel Macron, at once the fresh face of French politics and a familiar product of the French system, is the assumption that his first-round electoral victory makes triumph in the second round inevitable.

It is not. Macron, a political neophyte, has work to do.

Marine Le Pen, who took 21.4 percent of the first-round vote to Macron’s 23.9 percent, is not the favorite, but she is plausible in a way her father Jean-Marie Le Pen was not when he was crushed in the second round of the 2002 election. Some 7.6 million French people voted for her, 2.8 million more than her father in the first round 15 years ago. Never before has the National Front, a racist party, taken more than 20 percent of the vote. That this result for Le Pen has provoked relief in some circles is a measure of her party’s steady advance.

Disruption is in the air, and Le Pen’s anti-immigrant, anti-European, nationalist agitation is its most powerful expression. The establishment parties of center-left and center-right have been blown away, and with them some of the essential fabric of the Fifth Republic.