Acting decisively in a crisis is seen as one of Mr. Sarkozy’s virtues, and the events of the last 10 days in Toulouse have allowed him to remind the French of what they like about him, rather than what they dislike.

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Mr. Sarkozy is likely to gain a few points in the opinion polls, Mr. Perrineau said, and may reach his goal of beating Mr. Hollande in the first round of the election on April 22, when 10 candidates are competing. Among those are Ms. Le Pen, the centrist François Bayrou and the far-left candidate Jean-Luc Mélenchon, who is also squeezing Mr. Hollande.

“But the second round is much harder for Sarkozy,” Mr. Perrineau said. He believes that Mr. Hollande, who has a still-comfortable lead against Mr. Sarkozy in a head-to-head race, will win the presidency in the runoff on May 6.

Christian Salmon, writing in the magazine Marianne, asked, “Can one appeal to national unity, to respect, to healing when one has inflamed passions and divided the French?” He then described the way this national drama had been used “to capture the emotions of voters for electoral ends.”

The Toulouse killings allowed Mr. Sarkozy “the means to place himself in the heart of the campaign and reduce his adversaries to silence,” Mr. Salmon wrote. While the campaign was formally suspended, “we could only hear him — he was the only one able to campaign,” albeit through his office.

While Mr. Sarkozy remained aloof, his lieutenants attacked Mr. Hollande and Mr. Bayrou as having politicized Toulouse. Mr. Hollande had urged politicians to watch their “vocabulary,” while Mr. Bayrou had criticized the harsh anti-immigrant statements of Mr. Sarkozy and Ms. Le Pen as a contributing factor “to a growing climate of intolerance” that led to the singling out of minorities.

After Mr. Merah turned out to be the killer, however — the very kind of radicalized child of immigrants that the right and far right have warned about — the foreign minister, Alain Juppé, called such insinuations of politicization “ignoble,” adding, “Let’s not add the disgusting to the horrific.”

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Mr. Sarkozy’s tough-talking party leader, Jean-François Copé, criticized “the permanent ambiguity” of the Socialist Party, citing its abstention on the vote to ban the full-face veil in 2010 and what he called Mr. Hollande’s failure to make security a priority.

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Mr. Hollande took the high road, but his spokeswoman, Delphine Batho, said that Mr. Copé should be ashamed. “Shame on those who polemicize at the very moment that police intervened and were wounded,” she said, while a former Socialist prime minister, Laurent Fabius, promised, “We will be hard on crime, and I weigh my words.” Pierre Moscovici, head of the Hollande campaign, said he was “shocked by the indecency of the behavior” of Mr. Copé “in a tragic moment for the whole nation.”

These have not been comfortable days for Mr. Hollande, who has called for unity, for popular support for the police and for sympathy with the victims, both Muslim and Jewish. On Thursday, Mr. Hollande issued a news release, hailing “an end to insufferable anguish” in Toulouse.

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He said that “this ordeal reminds us that the fight against terrorism is a combat of every moment and allows for neither laxity nor weakness.” The French republic is stronger than its enemies, he said, and can defend itself “without losing anything of its values against its worst adversaries.”

While sincere, his words paled against the image of the president, standing before the French and European flags in the gilded Élysée Palace, on national television, calling for national unity and announcing seemingly harsh new measures to crack down on extremism.

Mr. Sarkozy said he would prosecute people who regularly consulted jihadist Web sites or who traveled abroad for indoctrination. “Any person who habitually consults Internet sites that praise terrorism and call for hatred and violence will be punished under criminal law,” he said, as well as anyone who travels abroad for “indoctrination into ideologies that lead to terrorism.” He also announced that he was instructing the authorities to investigate the promotion of extremism in French prisons — hardly a new problem, since nearly every French proponent of fundamentalist or jihadi thinking has been radicalized in French prisons, which have, experts estimate, a sizable majority of Muslim inmates.

But Mr. Sarkozy’s speech was a reminder, as he prepared to return in earnest to the campaign Thursday evening, of the extraordinary powers of the French president, who is a kind of republican monarch. And who — to Mr. Sarkozy’s cost at the beginning of his presidency, when he earned a reputation for loving “bling-bling” and wealthy friends — is expected to act like one. After a couple of days off line, his campaign Web site returned on Thursday afternoon, and he went to Strasbourg for an election rally in the evening. The “president-candidate” is back, but the campaign has shifted.