BERLIN (Reuters) - Germany’s Social Democrats (SPD), junior partners in Chancellor Angela Merkel’s ruling coalition, said on Sunday they would launch a new push to legalise same-sex marriage in Germany, a move opposed in the past by Merkel’s Christian Democrats.

German Chancellor Angela Merkel (2nd L) speaks with parliamentary faction leaders Volker Kauder (L-R) of the Christian Democratic Union (CDU), Thomas Oppermann of the Social Democratic Party (SPD) and chairwoman of the parliamentary state group of the Christian Social Union party (CSU) Gerda Hasselfeldt during a session of the German lower house of parliament, the Bundestag, in Berlin, Germany, November 25, 2015. REUTERS/Fabrizio Bensch/Files

Thomas Oppermann, who heads the SPD’s parliamentary faction, told Der Spiegel magazine his party would raise the issue at the next meeting of the right-centre coalition, a move welcomed by the pro-environment Green party.

The issue could help the SPD differentiate itself from Merkel’s conservatives as the uneasy coalition partners ramp up campaigning ahead of Sept. 24 national elections.

Merkel’s Christian Democratic Union and its CSU Bavarian sister party edged ahead of the SPD in the latest poll conducted by Emnid for the German newspaper Bild am Sonntag.

The SPD, which saw a surge in the polls after it nominated former European Parliament President Martin Schulz as its chancellor candidate, is hoping to win enough votes to form a new government with smaller allies such as the Green party.

Oppermann told the magazine that the party would also press for rehabilitation and compensation of people charged under a law that criminalised homosexuality which was in effect in postwar West Germany until 1969.

One leading conservative lawmaker, Deputy Finance Minister Jens Spahn, is openly gay and also backs same-sex marriage, according to Der Spiegel.

Spahn, whom Finance Minister Wolfgang Schaeuble named as a possible future candidate for chancellor, told Bild am Sonntag that he and his partner would like to adopt children.

“I think we would be good, responsible parents,” Spahn told the newspaper. “But unfortunately my party is conservative in the wrong way there.”

Katrin Goering-Eckardt, head of the Green party in parliament, said her party would ask for public debate on the issue.

“For years, we’ve seen nothing but hot air from the conservatives and the SPD,” she said in a statement.

German Family Minister Manuela Schwesig, a Social Democrat, said legalising same-sex marriage would mark important societal progress. “It’s time for the conservatives to move on this issue. It must stop putting the brakes on modernisation,” she said in a statement, citing widespread support for such a move.

The coalition government in 2015 agreed to small changes in same-sex civil partnership rules, but staunch opposition from the conservatives prevented approval of gay marriage.

A study by Germany’s Federal Anti-Discrimination Agency in January found that 83 percent of Germans supported legal equality for same-sex marriage.

Thirteen European countries now recognise and perform same-sex marriages - Belgium, Denmark, Finland, France, Iceland, Ireland, Luxembourg, the Netherlands, Norway, Portugal, Spain, Sweden and Britain.