Henry Samuel writes: 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.