Projections show President Macron and his allies, including the new party of his former prime minister Edouard Philippe, could fall short of a majority of 289 by as many as 40 seats.
French voters go to the polls in the first of two rounds that will decide whether President Emmanuel Macron gets a working majority in parliament or ends up without the support needed to drive through his reform agenda.
Less than two months after winning re-election, Macron faces a strong challenge from a united left-wing bloc that polls show could deprive the president of an outright majority even if it does not take control of parliament.
Government insiders expect a rather poor showing in Sunday's first round for Macron's coalition "Ensemble", with record numbers of voters seen abstaining and anger at the rising cost of living.
Hard-left firebrand Jean-Luc Melenchon's bloc hopes to capitalise.
At risk is Macron's ability to pass his reform agenda, including a pension reform he says is essential to restore public finances. His opponents on the left instead are pushing to cut the pension age and launch a big spending drive.
"We expect a difficult first round. Voters will want to send a signal," a government source said.
"But we're counting on the second round to show that Melenchon's programme is fantasy."
Initial projections immediately after the presidential election showed Macron was on course to get a majority in parliament, as is usual since the presidential term was shortened to five years.
But the president has kept a low profile since the vote, taking two weeks to form a government and only rarely making outings.
Meanwhile, Melenchon has successfully forged an alliance between his France Unbowed movement, the Socialists and the Greens, building momentum.
Projections now show Macron and his allies, including the new party of his former prime minister Edouard Philippe, could fall short of a majority of 289 by as many as 40 seats.
That would require him to reach out to competing political parties.
Some 14 of Macron's ministers are competing in local races and could lose their job if they fail to win a seat.
One cabinet member most at risk is Clement Beaune, Macron's Europe minister, who is campaigning in an eastern Paris constituency where the left has done well in the presidential election.
As a former adviser at the Elysee palace on matters such as Brexit, Beaune, 40, is a close ally of the president and has been one of a handful of aides who has gained a national and international profile.
"That would be a painful loss," a government source said.
Asked about the risk of defeat while he was campaigning in the Marais district of Paris, Beaune said: "In a democracy, universal suffrage is a risk by essence. Voters will decide."
On the other side of the political spectrum, polls show far-right leader Marine Le Pen could win a seat in her northern constituency straight from the first round by gaining over 50 percent of the votes.