PARIS (Reuters) - Close allies of the French president rounded on the opposition-led senate, warning it against abusing its power and seeking to undermine the presidency after it compelled Emmanuel Macron’s former bodyguard to answer questions over his violent conduct.

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Alexandre Benalla is under criminal investigation after he was filmed beating a protester while wearing a riot helmet and police tags, a scandal that has plunged Macron’s presidency into its gravest crisis since he won power in May 2017.

On Wednesday Benalla agreed to face questioning by senators after being told he risked prison if he refused to comply with the parliamentary inquiry.

Macron’s lieutenants have cautioned the Senate against over-reaching its powers and encroaching on a judicial investigation. Opponents of the president fired back, denouncing what they called an unprecedented attack on parliament.

“Any commission of enquiry that was to have political ambitions...and was to think it could use its role as a check on government to bring down a president would be violating the constitution,” Christophe Castaner, leader of Macron’s Republic on the Move party (LREM), told a news conference on Friday.

While the criminal probe is investigating Benalla’s actions during the May Day protest, the inquiry by the opposition-led Senate is looking into the Elysee Palace’s internal operations and whether it tried to keep the affair hushed up.

On Saturday, it was Macron’s justice minister who took aim at the senate committee: “It would be shocking if the executive were to interfere in a legal case. It would be no less so were parliament to do the same,” Nicole Belloubet wrote in Le Monde.

UNPRECEDENTED RIFT

Macron dismissed the Benalla affair as a “storm in a tea cup” when it erupted in July, but it has reinforced a perception among many French of a president who acts like a monarch and undermined his claim of building an “exemplary Republic”.

Benalla’s public questioning on Wednesday will keep the scandal in the public eye just as Macron tries to reverse a dramatic slump in popularity over the summer.

Jean-Luc Melenchon, leader of the far-left France Unbowed party, said Macron and his allies were locked in a battle with the Senate that was without precedent.

“By exporting its crisis of authority, the presidency is amplifying the turmoil and weakening the democratic institutions,” Melenchon wrote on his Facebook account.

At least two LREM senators sitting on the investigating committee have said they will not take part in Benalla’s questioning, French media reported.

The center-right party The Republicans, which controls the senate, has not recovered from Macron’s demolition of the mainstream parties in last year’s election, but denies it is seeking to make political gains from the Benalla scandal.

In a dig at parliament’s lower house, which is dominated by the ruling party and refused to launch an inquiry into the scandal, Senate President Bruno Retailleau said the upper house was conducting its work with “absolute rigor”.

“I want to tell Emmanuel Macron...that only Louis XIV believed that there was no one who could check his power,” Retailleau told Europe 1 radio.