Pascal Perrineau, an analyst from Sciences Po university in Paris, said neither Le Pen nor Macron now has even "the beginnings of a parliamentary majority".
Le Pen is a leading contender for France's presidential election, held in two rounds April 23 and May 7.
A second poll published on Friday showed a bigger, six-point gap separating the four main players in a first-round field of 11 candidates.
If elected president - a scenario analysts deem unlikely but not impossible - Le Pen would have immunity from prosecution.
Melenchon wants to impose a 100-percent tax rate on top earners, increase public spending by 173 billion euros over five years and renegotiate key European Union treaties.
With her trademark gravelly voice, the 48-year-old never hesitates to lock horns with journalists or rivals on televised debates as she takes on the political establishment. "It is up to us to say that emigration is always forced exile, a suffering".
Le Pen has denounced since the beginning a "political operation", mounted in particular, according to her, by former European Parliament President Martin Schulz. She had him expelled from the party over his views in 2015.
His line - "When we workers are summoned by the police, we do go there, we don't have workers immunity" - left Le Pen silent and was widely picked up on social media.
Lenders have refused to bankroll her run for the presidency, putting her at a financial disadvantage to other candidates.
The demand was made at the end of last month after she invoked her parliamentary immunity in refusing to attend questioning by investigating magistrates on 10 March.
"The caste which is screaming blue murder about me has forgotten it is supposed to be combatting Mrs Le Pen", Melenchon said.
She entered politics by joining her father's party at 18 in 1986.
PARIS (AP) French investigators are asking the European Parliament to lift far-right legislator Marine Le Pen's immunity so that she can face possible prosecution over suspected misuse of parliamentary salaries.