President Emmanuel Macron's centrists and the broad NFP left-wing grouping hope to stop the far-right from taking power in the lower house of parliament after it won roughly 33% of the vote at the first round last Sunday.
Macron's camp has started cooperating with the NFP, hopeful that tactical voting will prevent RN and certain aligned candidates from winning the 289 seats needed for an absolute majority.
The tactical withdrawals would be accompanied by cross-party calls for voters to back whichever candidate is best placed to defeat their local RN rival.
Macron decided to call a snap election after a poor showing in European elections last month, with RN gaining 31.4% of the vote compared with 14.6% for Ensemble.
The decision, which some analysts think may have been an effort to test the public — or RN's capacity to govern — is widely considered to have backfired.
While RN swept up a third of the first-round vote for the National Assembly, France's lower house, the left-wing New Popular Front garnered 28% and Macron's Ensemble managed just over 20%.
