The conservative candidate to succeed German Chancellor Angela Merkel failed to revive his campaign in a heated debate with his two main rivals on Sunday, according to a snap poll, as surveys show his party falling behind the center-left Social Democrats (SPD).

Social Democrat Olaf Scholz won the televised debate as a snap survey of voters by pollster Forsa showed 36 percent of voters polled believed Scholz won, ahead of 30 percent for Greens candidate Annalena Baerbock and 25 percent for conservative Armin Laschet. Merkel, a conservative in power since 2005, plans to step down after Germany's federal election on September 26.

Laschet, leader of Merkel's Christian Democrats (CDU), attacked the SPD's chancellor candidate for not ruling out a coalition with the far-left Linke party and sought to strike a chord with voters.

"I have felt headwinds now and again, as I do now," Laschet said in his closing remarks. "But aren't we all feeling the winds of change blowing us in the face? At times like these, we need steadfastness, dependability and an internal compass. That is what I offer." 

For much of the debate, Laschet traded barbs with Baerbock, who accused the CDU and SPD of doing too little to fight climate change, especially given devastating floods this summer.

"You obviously don't have a plan," Baerbock said of the other two, pledging to install solar panels on every roof and ban the sale of combustion engine vehicles from 2030. 

Scholz, who is the most popular of the candidates in polls, kept calm as the exchange became heated, focusing on financial topics such as taxes and pensions. He promised "a society that values respect. Respect for everyone." 

"And that is why we need better pay, a higher minimum wage, and of course also stable pensions," he said, adding, "We have to stop man-made climate change and ensure that we still have good jobs in 10, 20 and 30 years."

Support for the SPD rose two points from last week to 24 percent, their highest result in four years, according to an INSA poll conducted for newspaper Bild am Sonntag. The conservatives slipped a point to 21 percent, their lowest ever polled by INSA.