But a new survey showed support for the SPD hitting its lowest in nine months, underlining the challenge facing the party, which is in coalition with Merkel`s Christian Democrats (CDU) and their Bavarian ally the Christian Social Union (CSU).

Gabriel, who is economy minister and vice chancellor, also urged his party to push back against the rise of the far-right in Europe, citing in particular the historic win for France`s National Front in last Sunday`s regional elections.

"We Social Democrats are the stable factor in this federal government," he told an SPD party congress.

"Without us, the German government would already be paralyzed due to the argument within the CDU and CSU (over migrants) and our country would be in major difficulties."

Gabriel was referring to divisions in the conservative camp over Merkel`s open-door policy on migrants. About a million people are expected to have moved to Germany this year alone, fleeing conflicts and deprivation in the Middle East and beyond.

The CSU has challenged Merkel`s refusal to cap the influx, saying the government would lose popular support without such a ceiling. But Gabriel`s center-left SPD has so far failed to benefit from the infighting among its conservative rivals.

A poll from Forschungsgruppe Wahlen published on Friday showed support for the SPD on 24 percent, the lowest level in nine months and 15 points behind Merkel`s CDU/CSU conservative alliance.

Gabriel, 56, has a difficult job. As deputy chancellor in Merkel`s grand coalition, he has to show he is fit to lead the nation while also working out policies his party can agree to implement in tandem with the conservatives.

STANDING UP TO FAR-RIGHT

Looking ahead to the next federal election in 2017, some SPD members fear a repeat of the outcome in 2009 when the party`s vote slumped to a post-war low of 23 percent.

That followed a previous spell in a grand coalition, when Merkel took the credit for presiding over a robust economy despite the global financial crisis.

On Friday Gabriel rapped Merkel`s party for not pressing its French conservative ally Nicolas Sarkozy to team up with the Socialists to keep the National Front out of power in regional councils in France, where run-off votes will be held on Sunday.

"It is a shame for European conservatives - and for Germany`s CDU - that they allow this," Gabriel said.

"These movements run counter to everything we stand for and everything we represent," he added. "Let`s decisively oppose these enemies of Europe and these ideas against freedom -- that`s the most important job of all European Social Democrats."

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