French diplomacy could be plunged into an unprecedented period of turmoil if the far-right wins snap parliamentary elections, with President Emmanuel Macron and possible prime minister from her party National Rally (RN) to compete over who will speak for France.

The anti-immigrant, Eurosceptic party, National Rally of Marine Le Pen widened his lead over Macron’s centrist bloc, according to opinion polls released on Friday, the final day of campaigning ahead of Sunday’s first round of voting. The second round will be held on July 7.

Voters and diplomats have long been accustomed to the president making foreign and security policy decisions for France, a permanent member of the UN Security Council and a nuclear power with one of Europe’s most powerful armies.

But what is known in France, namely that the areas of defense and diplomacy ‘belong’ exclusively to the president, is not explicitly stated in the constitution, but is more of an unwritten rule, and this could cause problems if the National Rally party » form the next government.

During the three previous periods of “coexistence” since 1958, when the president and prime minister came from different parties, tensions sometimes flared but were quickly settled.

This time the controversy may prove more difficult, as o Jordan Bardellathe 28-year-old National Rally leader and likely next prime minister, has already said he would challenge Macron on global issues.

“It will be trench warfare,” he declared in the Reuters Gérard Petipre, French constitutionalist.

Even before the elections, the battle lines were being drawn.

Macron told leaders of the liberal Renaissance European Union group in Brussels on Thursday that he would nominate staunch ally Thierry Breton as France’s commissioner to the next EU executive, a diplomatic source said on condition of anonymity.

This came after Bardella said he was considering other candidates for the position. Le Pen decried Macron’s move.

“Emmanuel Macron expects a victory he cannot win. So he will not be able to name Mr Breton,” he told Europe 1 radio. “It is the prime minister’s prerogative to name the European Commissioner.”

Just an honorary title?

Experts say the French constitution, while giving the president greater powers over foreign policy than in most parliamentary democracies, still leaves the main levers of policy in the hands of the prime minister.

The president is commander-in-chief, presides over defense meetings, negotiates international treaties, and is ultimately the only person who can order the non-use of France’s nukes.

But the prime minister heads the French administration and controls the state budget, which must be approved by parliament.

That prompted Le Pen to say the president’s commander-in-chief title was “honorary”, angering Macron’s camp.

“It’s an honorary title because the prime minister holds the purse strings,” he said.

“Jordan is not going to pick a fight with him, but he has set red lines. In Ukraine, the president will not be able to send troops,” she added, referring to Macron’s refusal to rule out the possibility of sending French troops to Ukraine.

French constitutional scholar Petipre said it was within Le Pen’s rights to interpret the founding statute that way, but that it could eventually come back to haunt her, as a president has various ways of making life difficult for a government.

“She shot herself in the foot by signaling the start of a rough life together,” he said. “The president has extensive powers to resist, and now he will know he has to use them.”

Petipre cited the first period of cohabitation in 1986, when Socialist François Mitterrand refused to accept the foreign and defense ministers proposed by conservative Prime Minister Jacques Chirac because they had violently criticized the president. Chirac eventually had to propose two different names.

Weird duel: He who shoots first is dead

Previous symbioses were created for some mismatched scenes at global gatherings. Chirac insisted on attending the 1986 G-7 summit in Tokyo with Mitterrand, irritating the president, who nevertheless managed to be the only one to speak at the final press conference.

By the time of a European summit later that year, the two men had managed to agree on a common position even on divisive issues such as South African apartheid.

As part of a second symbiosis in 1994, the then president and prime minister agreed to send troops to Rwanda. The third and most recent symbiosis, with Chirac as president and Socialist Lionel Jospin as prime minister, also went relatively smoothly on foreign policy, with only a few disagreements.

The French would not be forgiving of leaders who exposed their divisions on the world stage, so both sides would have an interest in avoiding open hostility, said law professor Anne-Marie Coente of the University of Paris Pantheon-Sorbonne.

“Experts say that cohabitation is a strange duel in which the one who shoots first is dead,” he said.

However, previous symbiosis was between leaders from mainstream parties who largely agreed on major foreign policy issues and the importance of France’s alliances.

The National Rally party, which has never held power, has made no secret of its disdain for EU institutions and has said it wants France to leave NATO’s integrated command, though not while the war in Ukraine rages.

“If there is a symbiosis between Emmanuel Macron and the National Rally, it would not be like the previous ones … and it would be very harsh,” a former minister who lived through a period of cohabitation told Reuters on condition of anonymity.