Although campaigning officially started on Monday, the parties have been squaring up for the contest, which sees 6,500 candidates fighting for 577 seats in the National Assembly, since Macron's 7 June presidential victory.
Macron's election, LREM's creation and its subsequent tactics have placed the mainstream parties - the Socialists and the Republicans - in a quandary.
Socialists struggle to win voters
Some of their members have already broken ranks to become ministers, notably Prime Minister Edouard Phlippe, who was a Republicans member, while LREM is not standing candidates against about 50 others whom it judges most likely to break ranks.
As well as former prime minister Manuel Valls, who has already left the Socialists but not been granted full-scale LREM endorsement, they include former Socialist ministers Stéphane Le Foll, Marisol Touraine and Miriam El-Khomri, whose efforts at labour law reform Macron has pledged to continue as a top priority.
Although both Touraine and El-Khomri are officially Socialist candidates, their election material uses LREM's colours, presents them as pro-Macron candidates and makes little or no mention of their own party.
Former labour minister Miriam El-Khomri tweets her election poster
The remaining Socialist candidates now face the challenge of explaining to voters why they should back them against the party of the president, who was economy minister in the last, Socialist-led government.
Opinion polls show the Socialist Party (PS) in fourth place, with LREM in the lead, followed by the Republicans, the far-right National Front and left-winger Jean-Luc Mélenchon's France Insoumise.
PS National Secretary Jean-Christophe Cambadélis argued on Monday that his party was needed as a "counterweight" to Macron's supporters, pointing out that he had picked right-wingers as ministers for economic and social affairs and warning against forcing labour reform through without full consultation with the unions.
Right promise tax cuts
The Republicans (LR) have the same problem but the other way around.
Macron "has put his toe in the water by picking a right-wing prime minister ... Let's help take the plunge," Valérie Pécresse, the head of the Ile-de-France regional council, argued at the weekend.
Finding little to object to in Macron's economic programme, the mainstream right is trying to win support with the promise of a 10 percent cut in income tax and a pledge not to tax overtime pay, a measure introduced by Nicolas Sarkozy when he was president and scrapped by the Socialists.
They and their centrist allies are also warning against leaving the parliamentary opposition in the hands of "extremists", by which they mean both the National Front and Mélenchon's movement.
Prime minister calls for end to party haggling
Philippe, for his part, rejected the two traditional parties' arguments on Monday.
"The old partisan system with a right and a left who are systematically and diametrically opposed to each other is breaking up," he told regional paper Paris Normandie.
He called for a "progressive majority that will not be a prisoner of left-right divisions that have rather fossilised political life for years", warning that without it France would have to go back to "haggling between parties".