Donald Trump has said the Paris terrorist attack would boost Marine Le Pen's presidential chances after a last-minute poll gave her a modest increase in support.
The US president said the shooting would "probably help" Ms Le Pen in Sunday's election, because she is "strongest on borders, and she's the strongest on what's been going on in France."
"Whoever is the toughest on radical Islamic terrorism, and whoever is the toughest at the borders, will do well in the election," he said.
US presidents typically avoid weighing in on specific candidates running in overseas election. But Mr Trump suggested his opinion was no different from an average observer, saying: "Everybody is making predictions on who is going to win. I'm no different than you."
Cancelling visits and meetings on Friday, candidates traded blows across the airwaves as it emerged that the Isil-backed gunman had been kept in custody just 24 hours in February despite attempts to procure weapons to murder police.
Xavier Jugelé, 37, a policeman who had been deployed in the 2015 Bataclan attack, was killed in the shooting.
Ms Le Pen, the far-Right candidate, blasted the mainstream “naive” Left and Right for failing to get tough on Islamism, calling for France to instantly reinstate border checks and expel foreigners who are on the watch lists of intelligence services.
François Fillon, the mainstream conservative candidate, pledged an “iron fist” in the fight against “Islamist totalitarianism” – his priority if elected. “We are at war, it’s either us or them,” said the conservative, whose campaign has been weighed down by allegations he gave his British wife a “fake job”.
Meanwhile, Emmanuel Macron, the independent centrist, whom critics dismiss as a soft touch, hit back at claims shutting borders and filling French prisons would solve the problem, saying: “There's no such thing as zero risk. Anyone who pretends (otherwise) is both irresponsible and deceitful."
Sticking to his campaign agenda, far-Left candidate Jean-Luc Mélenchon told everyone to keep a “cool head” as he took part in a giant picnic.
A last-minute Odoxa poll taken after the attack suggested that Mr Macron was still on course to come first in Sunday’s first round, with Ms Le Pen just behind and through to the May 7 runoff.
However, Mr Fillon and Mr Mélenchon were still snapping at their heels.
The government on Friday announced elite units would join 50,000 police and troops to guard polling booths on Sunday in France's first presidential election to be held in a state of emergency.
Matthieu Croissandeau, editor of Nouvel Obs magazine, said the French are now thicker-skinned after two years of bloodshed. “The French are unfortunately getting used to terror attacks on home soil and I don’t think this latest one created the shock and awe that might have made a significant difference,” he said.
Ms Le Pen has struggled to get the campaign to focus on her party’s pet issues of security, Islam and immigration. By contrast, she has been thrown on the defensive over her position to pull out of the eurozone.
After the attack, she called on the “notoriously feeble” socialist, President François Hollande, to instantly reinstate border checks and expel foreigners who are on the watch lists of intelligence services. She said: “We cannot afford to lose this war. But for the past 10 years, Left-wing and Right-wing governments have done everything they can for us to lose it.
“We need a presidency which acts and protects us,” she said from her Paris campaign headquarters. Elected French president, I would immediately, and with no hesitation, carry out the battle plan against Islamist terrorism and against judicial laxity.”
But Ms Le Pen was not the only one to issue stern pledges. Mr Fillon, who also talks tough on security, said the fight against “Islamist totalitarianism” should be the next president’s priority.
“It will require an unyielding determination and a cool head,” the former prime minister said. “We are at war, there is no alternative, it’s us or them.”
Mr Fillon, though knocked off his initial course towards victory by incessant allegations involving “fake job” payments to his British wife, promised to govern with “an iron fist”.
But the moderate Mr Macron, whom other candidates have portrayed as too inexperienced, took a different tack, warning against any attempts to use the shooting for political gain. “I think we must once and for all have a spirit of responsibility at this extreme time and not give in to panic and not allow it to be exploited, which some might try to do,” he told French radio.
Ms Le Pen’s solutions, he said, were woefully simplistic and “would not protect France”. And he added that to promise a “zero risk” scenario “is both irresponsible and deceitful”.
He also took a swipe at Mr Fillon, saying that intelligence had been depleted on his watch as the prime minister in Nicolas Sarkozy’s presidency.
Meanwhile, Bernard Cazeneuve, the Socialist prime minister, accused Le Pen of “shamelessly seeking to exploit fear and emotion”.
Opinion polls have for months forecast that Le Pen would make it through to the run-off, but then lose in the final vote. Given the margin of error, none of the leading four candidates is a sure bet to reach the final. All hope to woo the third of French voters still undecided.
Previous terror attacks ahead of elections – such as the November 2015 attacks in Paris before regional ballots – did not effect those ballots.