BERLIN (Reuters) - Germany’s Social Democrats (SPD), stung by defeat in a state election on Sunday, fell to their lowest level in more than three months in a poll on Tuesday, underlining the scale of the task they face to oust Angela Merkel in a September election.

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The weak poll performance comes five days before a nail-biting weekend vote in North Rhine-Westphalia, home to almost a quarter of the German population, that is being widely seen as the biggest test of opinion before the Sept. 24 federal vote.

The loss in the northern state of Schleswig-Holstein, the second this year, has ratcheted up the pressure on the SPD.

The party saw a revival after choosing former European Parliament President Martin Schulz as leader at the end of January, but the ‘Schulz effect’ looks to be evaporating.

The SPD, junior partner in Chancellor Merkel’s right-left coalition, fell 1.5 percentage points to 27 percent in the INSA poll for Bild daily, its lowest level since late January, though still some 5 points above where it was before switching leader.

Merkel’s conservatives were up one point at 35 percent and the right-wing Alternative for Germany (AfD) also climbed one point to 10 percent.

The SPD has tried to blame local factors for Sunday’s loss in Schleswig-Holstein, including remarks the incumbent SPD state premier made about the break-up of his marriage, which some commentators said alienated female voters.

In North Rhine-Westphalia, charismatic local SPD state premier Hannelore Kraft is looking to hold onto power with the Greens, but polls show a very tight race.

On the national level, Schulz has emphasized fighting social inequality in his campaign but the party will only publish its policy manifesto in June. He said on Monday he would not make unrealistic tax-cutting promises but would lift investment.

He also made some comments which were widely interpreted as playing down his enthusiasm for a three-way leftist coalition involving the radical Left party, saying any coalition with him would be pro-European and protect Europe’s biggest economy.

SPD election promises so far, including extending some unemployment benefits, increasing childcare and infrastructure spending and cutting electricity tax, would cost up to 30 billion euros ($33 billion), Handelsblatt daily reported economists as saying.

The INSA poll put the Greens, the SPD’s preferred partner, on 7 percent, level with the pro-business Free Democrats (FDP), and the radical Left party at 10 percent.