“It does look like Sarkozy has an instrumental approach to what’s happened,” said Emiliano Grossman, a politics professor at Paris-based university Science Po. But Mr. Grossman said the president is drawing only a marginal benefit from the campaign’s new focus on security.

Despite his advance in the first round, he would still lose to Mr. Hollande in the May 6 run-off, by a wide margin. “The move is too obvious to work well,” said Mr. Grossman.