French Prime Minister Gabriel Attal, a close ally of President Emmanuel Macron, said he believed a cross-party bid to deprive Marine Le Pen's far-right National Rally of a majority in next Sunday's parliamentary run-off could succeed.
Attal was speaking after more than 200 candidates across the political spectrum agreed to pull out of local contests to clear the path for whoever was best placed to defeat the RN runner in their voting district.
"What these withdrawals show is that we can avoid an absolute majority for the far right," Attal told France Inter radio of the 289-seat tally the RN would require to control France's 577-seat National Assembly.
The Interior Ministry was due to release the candidate list for the second-round vote later but local media estimated up to 218 candidates had pulled out, reducing the number of districts in which the anti-RN vote risked being split.
Before the withdrawals, pollsters had calculated the first round put the RN on track for anything between 250-300 seats.
The question remains whether voters will go along with the effort to block the anti-immigrant, eurosceptic party. Centrist voters may baulk at supporting a far-left rival to the RN, while many left-wing supporters are so disillusioned with Macron that they will not stomach backing someone from his alliance.
Le Pen said on Wednesday that she could reach out to other parties if the RN falls short of an absolute majority. Her prime ministerial pick Jordan Bardella had insisted he would decline to form a government without a sufficiently strong mandate.
Financial markets were reassured by the bid to create a so-called "republican front" against Le Pen's party, lowering the risk premium on France's sovereign debt.
"The strategy would significantly limit the chances of Le Pen winning an outright majority," Jefferies chief Europe economist Mohit Kumar said in a note.
The RN came out well ahead in Sunday's first-round vote after Macron's gamble on a snap election backfired, leaving his centrist camp in a lowly third place behind the RN and a hastily formed left-wing alliance.
Le Pen has worked for years to soften the image of the RN but rights groups cite concerns about its "national preference" and anti-migrant policies, while its profound euroscepticism would pose serious threats to future European integration.
It has signalled plans to reverse Macron's reforms such as his unpopular move to increase the retirement age and economists question whether hefty RN spending plans are fully funded.
Yet even if the RN does not come to power, France could face months of political uncertainty through to the end of Macron's term in 2027 - when Le Pen is widely expected to mount a challenge for the presidency itself.
Asked about calls for the creation of a provisional cross-party government in the event of a hung parliament, Attal repeated his proposal for mainstream parties to cooperate in parliament on individual pieces of legislation.
"I hope the (Macron-allied) Ensemble camp is a big as possible. After that we will seek to secure majorities on a project-by-project basis."
The National Rally has attacked the cross-party bid to block it as anti-democratic and an attempt to undermine voters' wishes. Bardella told Le Figaro newspaper: "The real 'republican front' - that is us."
