Elections for the 577 seats in the lower house National Assembly are a two-round process. The shape of the new parliament will become clear only after the second round, a week later, on June 19.
The ballots provide a crucial coda to April's presidential election, when Macron won re-election and pledged a transformative new era after a first term dominated by protests, the Covid pandemic and Russia's war against Ukraine.
Stepping into the fray on Thursday, Macron acknowledged the stakes were high, warning France against choosing "extremes" which would add "crisis to crisis".
"If the presidential election is crucial, the legislative election is decisive," he said on a visit to the rural Tarn region, calling for a "strong and clear majority".
If the president's centrist alliance Ensemble (Together) retains an overall majority, he will be able to carry on governing as before.
Falling short could prompt a coalition with right-wing parties and an unwanted cabinet shuffle only weeks after the government was revamped.
A win by the left-wing alliance -- seen as unlikely by analysts but not impossible -- would be a disaster for Macron.
It would raise the spectre of a clunky "cohabitation" -- where the prime minister and president hail from different factions -- of the kind that has paralysed French politics in the past.
Left-wing leader Jean-Luc Melenchon, a former Marxist, has already made clear his ambition to become prime minister and stymie Macron's plan to raise the French retirement age, although the president would retain control over foreign policy.
While Macron and his European Union allies breathed a heavy sigh of relief after his solid if unspectacular presidential victory against far-right leader Marine Le Pen, the last weeks have brought no sense of a honeymoon.
Energy and food prices are soaring in France as elsewhere in Europe, the treatment of English fans at the Champions League final in Paris damaged France's image abroad and Macron has been accused by Ukraine of being too accommodating to Russia.
His new disabilities minister Damien Abad faced two rape accusations -- which he vehemently denied -- while new Prime Minister Elisabeth Borne has yet to make an impact.
Meanwhile, the French left has moved on from the disunity that saw it fail to make the presidential election run-off by forming the NUPES alliance, which groups Melenchon's hard-left France Unbowed party, the Socialists, Greens and Communists.
It is mounting an increasingly serious challenge to Macron's own alliance, though the two-round system and an abstention rate predicted to reach record levels of well over 50 percent could both play into the hands of the president.
The Together alliance and NUPES will be neck-and-neck in terms of the popular vote on around 28 percent in the first round, polls show.
"The day after Emmanuel Macron's re-election, his lieutenants saw the legislative elections as a formality," said leading French daily Le Monde.
"But now they have seriously lowered their ambitions... Even a defeat, unthinkable several weeks ago, is now considered not impossible at the Elysee," it said, referring to Macron's office.
Macron has made clear that ministers who are standing in the election -- including Borne, who is making her first attempt at winning a seat -- will have to step down if they lose.
'Majority not assured'
Of the 577 deputies in the National Assembly, eight represent France's overseas territories and 11 account for French nationals living abroad.
Macron's party and his allies currently hold an absolute majority of 345 seats.
The latest opinion poll by Ipsos projects that Macron's alliance would win 275 to 315 seats. This means it is by no means assured of an absolute majority, for which 289 seats are needed.
"Projecting seats is a perilous exercise at this stage," said the managing director of Ipsos France, Brice Teinturier.
"An absolute majority (for Together) is not assured but the presidential majority does have a certain margin" that will play to its advantage in the second round.
Under France's system, a candidate needs over half the vote on the day as well as the backing of at least 25 percent of registered voters in a constituency to be elected outright in the first round.
Otherwise the top two candidates in a constituency, as well as any other candidate who won the backing of at least 12.5 percent of registered voters, go forward to the second round, where the candidate with the most votes wins.