Political party rivals of Chancellor Angela Merkel are meeting in Berlin on Tuesday in a bid to forge a new leftist coalition aimed at toppling her conservative Christian Democrats (CDU) from office at next September's general election.

About 90 members of the Social Democrats (SPD), the hard-left Die Linke and the environmentalist Greens are meeting to assess the chances of teaming up in a new three-party coalition to head off Merkel possibly winning a fourth term as head of Europe's biggest economy.

"We do not want to decide anything concrete. It is all about an exchange concerning the situation," said Die Linke parliamentary leader Jan Korte. "It's about seeing what can come out of it."

Merkel has still not officially announced whether she will head the CDU's election campaign.

An announcement is expected closer to the annual CDU party conference in December, where she is likely to face a vote as party chief.

The left-leaning SPD, which is at present the junior member of Merkel's ruling coalition, has also been gripped by speculation about who will be the party's so-called chancellor candidate at the election, still 11 months away.

Last week's voter survey from Berlin pollsters Forsa showed the SPD gaining 22 per cent support, with the Greens garnering 11 per cent and Die Linke on 10 per cent.

But in line with other surveys, the Forsa poll showed the CDU and its Bavarian-based Christian Social Union (CSU) allies remaining the biggest party in the lower house of parliament, the Bundestag, with 34 per cent of the vote.

However, the political landscape and the building of coalition governments in Germany has dramatically changed since the last election in 2013 following the rise of the right-wing populist Alternative for Germany (AfD).

Founded just three years ago, the AfD has successfully capitalized on the unpopularity of Merkel's liberal refugee policy in parts of the electorate and appears set to enter the Bundestag next year.

Still, polls also show Merkel coming in far ahead of SPD chief and Minister for Economic Affairs Sigmar Gabriel in two-way contest between the two leaders.

As a result, some SPD members have begun to consider Gabriel's party colleague and President of the European Parliament Martin Schulz as a possible candidate to take on the chancellor, should she indeed decide to run as many political analysts expect.

The failure last week of the SPD and Die Linke to reach an agreement on a common nominee for next year's vote on the largely ceremonial post of German president also underlines the difficulties facing the three parties in forging a so-called red-red-green government to end the CDU's reign.

There are also differences between the three parties on other issues, including Germany's involvement in international military operations.

In addition, Merkel and the CDU are considering their coalition options for after the election with her conservatives having been wooing the Greens as possible new partners.

Polls also show the pro-business Free Democrats, a one-time CDU coalition partner before they were ousted from the Bundestag at the last election in 2013, as possibly returning to parliament after next year's ballot.

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