BERLIN (Reuters) - Germany's parliament is set on Friday to vote to allow marriages between people of the same sex, embarrassing Chancellor Angela Merkel's conservatives and handing her Social Democrat (SPD) challengers a rare victory months before a national election.
Success in passing the so-called "marriage for all" amendment will be a tonic for the centre-left party, which has barely made a dent in Merkel's comfortable poll lead after months of campaigning.
At a parliamentary group meeting on Wednesday, Merkel accused the SPD of "ambushing" her by bringing forward a vote on an issue that divides her conservative bloc down the middle.
"It's sad and completely unnecessary ... that such a decision has turned into a political confrontation at the very moment when there was a realistic outlook for a process that could have crossed party lines," Merkel told Wirtschaftswoche magazine in an interview.
"Every member of parliament should be able to follow their conscience."
When Merkel announced on Monday that she would make any vote on equal marriage a matter of conscience for her lawmakers, freeing them from the party whip on the matter, she was hoping to shut down a line of attack that was a threat to her party.
With same-sex marriage now legal in most of Germany's neighbours and backed by all parliamentary parties other than her conservative bloc, continued opposition to change risked looking anachronistic.
But immediately after her decision, SPD leaders moved to bring a vote to the floor of the full parliament, arguing that by making it a matter of conscience Merkel had released her coalition partner from its commitment not to hold a vote.
Merkel's volte face drew the ire of some of her own lawmakers, who usually defer to a leader who has led them to three election victories and still enjoys a 14-point poll lead over the SPD.
"Are we going to change our view whenever it is politically convenient?," asked lawmaker Wolfgang Bosbach in the Stuttgarter Zeitung newspaper.
Together, the pro-equal marriage SPD, Greens and Left parties command a slim majority in parliament, meaning the amendment is likely to pass even without the expected support of many conservative lawmakers.
The proposal, moved in 2015 in the upper house of parliament by the state of Rhineland Palatinate, would, if passed by the lower house, be signed into law by the president some time after July 7.
Victory would give a boost to the SPD's flagging morale. But with three months of campaigning to go until the September 24 parliamentary election, Merkel's early retreat may prove to have been the better strategic choice.
Reporting By Thomas Escritt; Additional reporting by Andrea Shalal; Editing by Richard Balmforth