Chancellor Angela Merkel could suffer an embarrassing defeat in the Bundestag on 18 April if members of her centre-right coalition break ranks and back an opposition bill requiring German companies to hire more women executives.
The threat of a rebellion is looming just five months before Germany holds an election in which Merkel will be trying to win a third term. Among the conservatives who may vote with the opposition is Labour Minister Ursula von der Leyen, who says voluntary pledges to appoint more women have proven a failure.
The measure that would require Germany's 30 largest listed DAX companies to allot 40% of their supervisory board seats to women by 2023 originated in the upper house of parliament, where the opposition Social Democrats (SPD) and their Greens allies secured a majority earlier this year.
With the election looming and Merkel's party far ahead in the polls, the centre-left opposition is looking for ways to upset her campaign.
In addition to the women's quota, the left is planning to use its new majority in the Bundesrat to force votes on other issues that divide conservatives, including equal tax treatment for gay couples and subsidies for stay-at-home parents.
Merkel's centre-right government would be defeated on the female quota issue if 21 or more ruling party lawmakers vote with the opposition.
"We're not going to let them divide us," said Volker Bouffier, the CDU premier in the state of Hesse, who opposes the quota. CDU deputy party leader Julia Kloeckner, another quota critic, denounced the looming vote as an election year ploy by the opposition.
"Why should we fall for that trick?" she said.
Explosive issue for Merkel
In the lower house, a group of 25 coalition deputies have signed a "Berlin Declaration" supporting the introduction of the quota, which in a first step would require that women hold one out of every five supervisory board seats by 2018.
Whether all 25 withstand the coalition pressure and vote with the opposition remains to be seen.
Merkel allies, led by parliamentary floor leader Volker Kauder, were applying pressure on von der Leyen and other deputies who have spoken out in favour of the quota for women.
Von der Leyen, one of the few members of the CDU who has openly challenged Merkel on policy issues in recent years, has long supported the so-called "Fraunquote" but has kept mum about how she will vote on Thursday. She is seen as the pivotal voice and all eyes are on her this week.
"It's a very explosive issue for the coalition," said Thomas Jäger, a political scientist at Cologne University.
"It's rather remarkable that von der Leyen has gone out so far on a limb like this," he added. "She's got higher ambitions and this will clearly damage her in the CDU. It seems the conservatives in the party have had to swallow so much drift to the left over the years that they're drawing a line in the sand on this. Not an inch further!"