BERLIN: Germany's Social Democrats (SPD) have opened up a bigger lead over Armin Laschet's conservative bloc, a poll showed on Sunday ahead of a primetime television debate between the three main candidates to succeed Angela Merkel as Chancellor.

Two weeks before a national election that has proved to be unexpectedly turbulent, the INSA poll for Bild am Sonntag put the centre-left SPD on 26%, up a point from a week ago and their highest rating since June 2017.

Laschet's conservative bloc was unchanged at 20% and the Greens were down 1 point at 15%.

Despite losing 8-9 percentage points in the last three months, Laschet told Deutschlandfunk (DLF) radio the conservatives had not given up, insisting there was still time to win.

"A lot of people have not yet decided," Laschet said. "This is a decisive election and we are fighting for a majority on Sept. 26," he said.

The conservatives are already missing the pulling power of Merkel, who after four election victories and 16 years leading Europe's biggest economy is not running again.

This week's edition of the influential Spiegel magazine has a front cover picture of Laschet holding his hands in front of his mouth with the headline "Oooops".

The jovial if uncharismatic leader of Germany's most populous state has made mistakes during the campaign, including being caught on camera laughing during a visit to a town hit by lethal floods in July. He apologised.

The TV debate starting at 8.15 pm local time (1815 GMT) will offer Laschet an opportunity to appeal directly to voters.

Laschet has started targeting the SPD's chancellor candidate directly and said Olaf Scholz could form an alliance with the Greens and the far-left Linke that would raise taxes and make foreign policy mistakes.

"This would hurt Germany and its affluence. We want to modernise Germany and also preserve stability," Laschet said in the radio interview.

Scholz, who is not keen on sharing power with the Linke but has not ruled out working with them, won the first televised debate two weeks ago according to polls and is still by far the most popular of the candidates for chancellor.

His ratings have so far not been hit by last week's raids on the finance and justice ministries as part of an investigation into the government's anti money-laundering agency, which as finance minister, Scholz oversees.

Most experts think a three-way coalition is the most likely outcome of the election - a scenario that could take several months to negotiate during which time Merkel remains chancellor.