Former Chancellor Gerhard Schroeder, the last Social Democrat (SPD) to lead Germany, urged his party to go out and fight for victory in the September election despite trailing Chancellor Angela Merkel's conservatives by 15 points in polls.
Speaking at a forum Monday evening, Merkel said she was open to a vote of conscience on the issue, in which lawmakers are freed from toeing the party line - and in which the reform would nearly certainly pass.
Civil partnership has been allowed since 2001, but Merkel's Christian Democratic Union has resisted calls to open up marriage to same sex couples.
Merkel's shift comes as polls show that large majorities of German voters, and all other mainstream parties, are in favour of allowing same-sex marriage, which has been legalised in many other major Western democracies in recent years.
He also accused Dr Merkel of failing to stand up against US President Donald Trump, and sharply criticised Turkish President Tayyip Erdogan, saying he should release jailed journalists.
Martin Schulz, leader of the Social Democratic Party (SPD) - the second largest party in Parliament - announced he will be calling for a vote in the next few days. A YouGov poll conducted in May found 66 percent of almost 1,110 respondents were in favor of legalizing same-sex marriage, with 57 percent saying that adoption by gay couples should be allowed.
In almost 12 years as chancellor, Merkel has moved her party to the center and away from conservative orthodoxy, speeding up Germany's exit from nuclear power and ending military conscription among other moves.
The chances of denying Merkel a fourth term look slim for the SPD, who are now junior partners in her right-left coalition.
Schulz's call for a more equal society may find resonance with the working poor, but Koecher noted that with Germany's solid economic outlook and record low unemployment, "it is likely to have limited impact".
The chancellor said she had not had time to take up the invitation, but she used it as a way to illustrate that it may often be better for children to live permanently with a loving couple no matter what their sex, rather than moving from home to home in foster care.
If the vote were conducted without party pressure, it is expected that a marriage equality law would easily be passed.
"For me, personally, marriage is a man and a woman living together", she said in 2015. Some 40 percent of members who have joined since Schulz's nomination are under 35, says Jusos, the youth branch of the SPD.
Merkel chairs the CDU which together with its sister party, the Bavarian Christian Social Union (CSU), is known for socially conservative views and has held a longstanding opposition to the introduction of same-sex marriage.