German Chancellor Angela Merkel’s main election rivals are set to publish draft proposals on same-sex marriage.

At present, Germany is governed via a Grand Coalition comprised of the centre-left Social Democratic Party (SPD) and Angela Merkel’s right-wing Christian Democratic Union (CDU), along with the Christian Social Union in Bavaria.

Though same-sex couples are permitted to enter unions in the country, the government has ruled out any progress on equal marriage due to a strict coalition agreement.

However, the SPD – who are gaining on Merkel’s CDU in the polls ahead of September’s federal elections – are now challenging their coalition partners on the issue.

German newspaper Funke Mediengruppe reports that the SPD has drawn up a draft bill on the issue and officials are planning to raise it with Merkel’s party at this week’s Coalition meeting.

According to The Local, SPD Bundestag leader Thomas Oppermann said: “In the future, marriage should be possible as well for same-sex couples.

“I hope that the CDU and CSU can finally change their spots.”

The issue is likely to be a key dividing line between to the Coalition parties.

Ahead of the election, Merkel will be keen not to alienate her traditional right-wing voter base, which is already under pressure from the far-right AfD over her refugee policies. But she will also want to win over centre-ground voters who are supporting the SPD in greater numbers.

Chancellor Merkel has repeatedly ruled out calls to introduce equal marriage, saying: “For me, marriage is a man and a woman living together.”

The upper house of the German Parliament has previously passed symbolic motions calling for same-sex marriage.

Unlike the Bundestag (main chamber of the Germany Parliament), where Chancellor Merkel’s Grand Coalition holds control, the Bundesrat (equivalent to the Upper House/Senate) is controlled by the 16 state governments, with a left-wing majority.

A coalition of the Social Democrats, the Greens and the Leftbacked a bill in the Bundesrat that would recognise unions between same-sex couples, despite Chancellor Merkel’s insistence it is between a man and a woman.

However, no legislation is likely to pass the Bundestag without the government’s blessing

Germany allows same-sex couples to enter into registered life partnerships that provide some of the benefits of marriage – but the CDU and CSU continues to oppose same-sex marriage.