With his popularity at a record low, the Socialist President is on a collision course with militant cardinals determined to rouse France's Catholic masses against him.
Opponents of the reform threaten to bring a million protesters on to the streets of Paris in a rerun of the demonstrations that blocked president Francois Mitterrand's attempt to abolish Catholic schools in 1984.
Amid accusations of amateurism and a gaffe-prone style, Mr Hollande's popularity has sunk to 36 per cent in the latest TNS Sofres poll for Le Figaro magazine, a drop of five points in a month.
Six months into his presidency, he is the most unpopular French head of state at this stage of his presidency since the Gaullist Jacques Chirac in 1995, who faced widespread labour unrest and a general strike.
During a ceremony at the Elysee Palace, Mr Hollande, who was raised
by devout Catholic parents but is described as "following no faith
whatsoever", whispered to the gay rights campaigner Jean-Luc Romero
about his Catholic critics: "They're tough right now ... "
Cardinal Andre Vingt-Trois, the Archbishop of Paris, has lobbied Mr Hollande and Jean-Marc Ayrault, the Prime Minister, to drop the reform, which will be unveiled by cabinet on Wednesday.
When lobbying failed, Cardinal Vingt-Trois ordered his bishops and priests to read out a prayer at mass in all French churches to defend the idea of marriage as always between a man and a woman.
Last week, Cardinal Vingt-Trois said during a mass for parliamentarians that they must follow their conscience on marriage reform, and he urged them to rebel against the party line.
Cardinal Philippe Barbarin, Archbishop of Lyon, warned that gay marriage would prove to be a slippery slope. "After (gay marriages), they'll want to allow couples with three or four people. Then, one day perhaps, the ban on incest will be abolished," he said.
Campaigners believe a show of force in the street might make Mr Hollande cave in.
Jean-Francois Cope, a candidate for the leadership of the centre-right UMP party of former president Nicolas Sarkozy, said if he won the contest he would call for demonstrations to protect "the future of French children".