World Bulletin / News Desk

Germany's Social Democrats Tuesday started a membership ballot on whether to again govern under Chancellor Angela Merkel's conservatives after an opinion poll showed them trailing the far-right AfD party for the first time.

If the rank-and-file of the 153-year-old labour party give the thumbs up -- with the results to be announced on March 4 -- Merkel is set to launch her fourth-term government within a few weeks.

If they vote 'no' in the postal and online ballot, Europe's biggest economy faces more political paralysis and likely snap elections that would threaten a speedy end to Merkel's 12 years in power.

Despite weeks of turmoil and bitter infighting, the SPD leadership is hopeful its members will back the proposal for a continuation of the current right-left "grand coalition" government, known in German as the "GroKo".

But the outcome is far from certain, given the volatile mood in the party which scored its worst post-war result of just 20.5 percent in the September 24 election.

The SPD's youth and left wings are driving a passionate #NoGroKo campaign, arguing that the party must rethink what it stands for and rebuild as a combative opposition force.

The SPD's credibility has been badly bruised by a series of U-turns, which last week saw election loser Martin Schulz resign as party leader after less than a year in the post.

The party's ratings are in freefall, with the latest polls giving it just 15.5 percent support -- narrowly behind the far-right, anti-immigration Alternative for Germany (AfD) party.

Merkel's conservative CDU/CSU bloc remained the strongest political force with 32 percent support.

The AfD cheered having polled as Germany's second strongest party, and its parliamentary leader Bernd Baumann vowed the populists were now training their sights on "our next target, the CDU".