"The path to equality is open," added the chairwoman, Renate Künast of the left-leaning ecologist Greens party, in a tweet on Wednesday.
The Social Democrats (SPD), junior coalition partner in Angela Merkel's “grand coalition” took the unusual step of allying itself with the Green Party and Die Linke (the Left Party) to get the votes necessary to put the bill on the agenda.
Friday is the last day of the current legislative period before the Bundestag (German parliament) breaks up for the summer. The bill is expected to pass comfortably, with the support of the SPD, the Greens and Die Linke.
The reform would grant full marital rights - including the possibility to jointly adopt children - to gay and lesbian couples, who in Germany are now only able to enter so-called civil unions.
But the decision by the SPD to push for a vote this week has caused a serious conflict within the coalition government, as Merkel's conservative Christian Democrats (CDU) opposed holding a vote before the national election on September 24th.
The SPD meanwhile upped the ante on Wednesday, calling for a “named vote” in the Bundestag, whereby MPs would not vote anonymously.
“That'll be a huge problem for the CDU,” party faction leader Thomas Oppermann said, referring to the fact that many CDU MPs oppose same-sex marriage legislation.
Volker Kauder, a senior ally of Merkel in the CDU, accused the SPD of a “breach of trust”. Patrick Sensburg, another CDU parliamentarian, claimed the SPD was deliberately trying to bring down the coalition, only weeks before the national election.
On Monday, Merkel said she was open to a vote of conscience on the issue of gay marriage, in which lawmakers are freed from toeing the party line.
Previously, Merkel had repeatedly voiced her personal reservations about full marriage rights, including joint adoption, for same-sex couples, citing "the well-being of the children".
The sudden change is position from Merkel caught many in her party by surprise, and led to an angry backlash by MPs who accused her of riding roughshod over their interests.
“We are totally fed up with her,” one CDU MP told the Passauer Neue Presse after a meeting of MPs on Tuesday in which Merkel confirmed she would allow for a free vote on a gay marriage bill.
Conservatives within the CDU also warn that a gay marriage bill would require a change to the German constitution.
“The interior and justice ministries have always been of the opinion that same-sex marriage can't happen without a change to the constitution,” Günter Krings (CDU) told the Rheinische Post.
“There is a lot of evidence to suggest that the proposed change to the law will contravene the constitutional definition of marriage.”