By Madeline Chambers
BERLIN (Reuters) - A no-nonsense cleaning lady who humiliated the leader of Germany's Social Democrats (SPD) on camera by accusing him of selling out to Chancellor Angela Merkel's conservatives has galvanized a party rank-and-file desperate for a leftward shift.
Sigmar Gabriel, dogged by ill health and forced to quash rumors he will quit, is under unprecedented pressure but moving left would ratchet up tensions within Germany's ruling coalition where the SPD is junior partner to Merkel's conservatives.
With the conservatives also split on how to respond to a surge in support for right-wing populists, several major policies - ranging from a U.S.-EU trade deal to energy laws and steps to deal with the influx of more than one million migrants over the past year - are at stake.
At a party event this week, union official Susanne Neumann accused Gabriel - who is vice-chancellor and economy minister - of ignoring the problems of ordinary Germans struggling to find secure work and earn a decent wage. She made short shrift of his case that it was the fault of Merkel's conservatives.
"So why do you stay with the conservatives?" asked Neumann to loud cheers and applause from the audience.
Gabriel's argument that he can achieve more in government than in opposition has fallen on deaf ears as his party languishes at historic lows around 20 percent and the right-wing Alternative for Germany (AfD) gains from the migrant crisis.
This week's shock resignation by Austrian Social Democrat Chancellor Werner Faymann due to splits in his party and the impact of the migrant crisis has also shaken the Germany's SPD.
"What is tearing apart Austria's Social Democrats also threatens the SPD. For every migrant who arrives in the country, the Social Democrats lose voters," wrote Die Welt daily.
'NO MORE MERKEL'
Neumann's solution, to ditch the grand coalition, has drawn support. It spurred Matthias Miersch, head of leftist lawmakers in the SPD, to warn in Bild daily that the Austrian experience showed the results of a 'grand coalition' becoming the norm.
"We must get out of the grand coalition after the next parliamentary election - that is clear," Miersch told Bild.
There is no sign that Gabriel will walk out of government and experts say he would be foolish to rule out coalition options. Instead, he seems to be stepping up the rhetoric to keep the left on side.
In the last week alone, Gabriel has played to his party base by raising doubts about a U.S.-EU trade deal and by striking a different tone from conservative Finance Minister Wolfgang Schaeuble on Greek debt relief.
Other policies which could be hit by coalition strife are an integration law which still has to go to cabinet and plans to cut CO2 emissions and possibly exit coal. Ties with Turkey, a key partner in the migrant crisis, are also a flashpoint.
There are also questions about whether Gabriel, 56, is the right man to lead his center-left party into next year's federal election against Merkel, who remains popular despite waning support for her conservative bloc.
Though a rousing speaker, Gabriel has a reputation for being unreliable and changing his mind. A bout of shingles which forced him to cancel a high-profile trip to Iran seemed to stoke speculation about his future at the weekend.
Although the speculation is damaging, jettisoning Gabriel now could backfire as it would smack of "last-minute panic", said Oskar Niedermayer of Berlin's Free University.
(Additional reporting by Thorsten Severin; Writing by Madeline Chambers; Editing by Gareth Jones)