A new alliance of the French left is putting pressure on Emmanuel Macron’s grouping in the second round of the parliament election, as the president tries to hold on to his parliamentary majority.

Macron’s centrists could lose dozens of seats in France’s national assembly in the final next Sunday after a strong showing by a historic alliance of parties on the left, led by the hard-left Jean-Luc Mélenchon’s France Unbowed with the Socialists and the Greens.

“A few months ago, there were TV debates about whether the left was dead and finished,” said Clémentine Autain, who is likely to win re-election in Seine-Saint-Denis outside Paris for the alliance known as the New Popular Ecological and Social Union or Nupes. “Frankly, this alliance is a success.” She said it was now crucial for the left to try to bring out the young people and those on low incomes who did not turn out to vote.

Macron, who was re-elected president in April against Marine Le Pen, needs a majority for his grouping in the lower house of parliament in order to have a free hand for his proposals to raise the retirement age, cut taxes and make changes to the welfare system.

Macron’s centrists and their allies, running under the banner of Ensemble (Together), are still predicted to win the greatest share of seats in the 577-seat house. But to secure an absolute majority, they need 289 seats. Pollsters predict them taking between 255 and 295 seats, meaning a clear majority is not guaranteed.

Le Pen’s far-right National Rally also fared well and is likely to increase its seats, despite a historically low turnout of only 47% across France.

A result on Sunday without an absolute majority for Macron’s camp could lead to a messy scramble as it attempts to broaden its centrist alliance to new parties or do individual deals for every piece of legislation, for example with rightwing opponents such as Les Républicains. There could also be a government reshuffle.

Macron’s centrists fared worse in Sunday’s first-round parliamentary vote than they had done when he was first elected five years ago in 2017.

“It’s clearly not a great result,” said Mathieu Gallard, the research director of the pollsters Ipsos, on France Inter radio. “Macron’s grouping has also lost 3.9m votes from his first-round score in the presidential election.”

The shape of the future parliament will now depend on the final vote on 19 June.

Several of Macron’s ministers are fighting close constituency battles against the left and will have to quit the government if they lose. They include Amélie de Montchalin, the environment minister, who is struggling in Essonne, just outside Paris. Macron’s party leader, the civil service minister, Stanislas Guerini, as well as the Europe minister, Clément Beaune, are fighting close races in different areas of Paris.

Montchalin set the tone of a heated battle against the left, telling a TV station that the left alliance were extremists who wanted “anarchy” and “disorder” and to weaken French institutions. Élisabeth Borne, the prime minister, who is expected to be elected in Normandy, claimed Mélenchon’s left alliance stood for a “rupture with Europe, a fascination for authoritarian régimes” and alignment with Russia.

The Nupes scored a tactical coup in the first round. The fact that they joined forces for the first time in 25 years, fielding a single candidate in constituencies, means they will vastly increase their representation in parliament. They will more than double their number of seats and are predicted to win between 150 and 210 to become the biggest opposition force in parliament.

Four members of parliament for the left, including three young female candidates, were elected outright in the first round in the Paris area, as the alliance gained ground in and around urban centres.

Meanwhile, the progress of Le Pen’s party was significant. It increased its votes by more than 1.2m from the last parliamentary election in 2017.

The far right could win up to 40 seats, compared with eight won in 2017. This would be historic, because Le Pen’s party has traditionally been held back by a lack of proportional representation in parliament and has struggled in the first-past-the-post voting system. More than 15 seats would give Le Pen a formal group in parliament, with greater visibility to speak and put issues on the agenda, as well as crucial extra financial resources. In some départements, such as the Pas-de-Calais in the north and the Vaucluse in the south, the party made it through to the second round in every constituency. It also gained support in the north-east.

The rightwing party Les Républicains and its allies came fourth in terms of vote share and hope to hold on to between 33 and 80 seats. This is a drop from more than 300 members of parliament 20 years ago under Jacques Chirac, or the 100 seats held by the right and its allies at the end of the last parliament. But the group could play an important role if Macron’s centrists fail to get a majority and seek new alliances.