The Socialist Party is divided as never before by Prime Minister Manuel Valls' and President François Hollande's plan to amend France's constitution to include the right to strip binationals of French citizenship if they commit a "crime that constitutes a serious violation of the life of the nation".
Opponents, including former prime minister Jean-Marc Ayrault, former labour minister Martine Aubry and Paris mayor Anne Hidalgo, say it is unjust because it can only apply to one part of the population and pointless because it is unlikely to deter suicide bombers.
Now a group of party members, including two members of the National Committee, are calling on the ethics committee to rule that it is against the party's principles of equality of all citizens and opposition to discrimination.
The chair of the committee is lawyer Jean-Pierre Mignard, usually an ally of Hollande, but he, too, has opposed the idea, suggesting instead that a punishment involving the loss of the vote and other rights that could apply to all.
That would require a three-fifths majority of both houses of parliament, sitting together in a special session.
Most, but not all, of the mainstream-right Republicans say they will support the move and the far-right Front National is very much in favour.
But the hard left is opposed and some opponents claim that more than 200 Socialist MPs could vote against.
Republican MP Henri Guaino on Tuesday called for a referendum on the question if it fails to win the necessary majority of MPs and senators.
Valls has lashed out at his critics twice in two days, rejecting the argument that he has stolen the idea from the far right and pointing out that it is already the law in Britain, Canada, Switzerland and the Netherlands.
Opponents are forgetting that France is at war with terrorism and that French citizenship is based on acceptance of the "republican ideal", he said on his Facebook page.
Deprivation of citizenship already exists as part of the civil code, having been introduced in 1848 to punish slave traders who refused to give up after abolition, as Valls has not failed to point out.
It was used in World War I, with more than 500 people who were originally from enemy powers losing French citizenship.
But, as Valls's opponents point out, it saw most use under the World War II Vichy regime, which collaborated with the Nazis.
It expelled 15,000 people, including some like General Charles De Gaulle who had been born in France, from the French nation.