BERLIN: The conservative candidate aiming to succeed German Chancellor Angela Merkel remained under pressure on Sunday ahead of a televised election debate that will be one of his last chances to catch up with Social-Democrat rival Olaf Scholz.

The latest INSA poll for Bild am Sonntag puts the Social Democrats (SPD) at 26% support, stable from a week ago, while the conservative bloc of Merkel's centre-right Christian Democratic Union and its Bavarian sister party, the Christian Social Union, added half a percentage point to come in at 21%.

The gap has been even wider in polls measuring the popularity of the individual chancellor candidates, indicating the uphill struggle conservative Armin Laschet is facing against Scholz ahead of next Sunday's general election.

Laschet has been under fire since he was caught on camera laughing during a visit in the summer to a flood-stricken town.

In an interview with Bild am Sonntag, Scholz, who serves as finance minister, pledged to keep pensions and retirement age stable, adding those would be red lines in any coalition talks.

"Everybody can rely on the fact that a government led by me will do just that," Scholz said.

Scholz and Laschet will clash on Sunday at 8.15 p.m. (1815 GMT) in what is the last of three primetime television debates that will also include Annalena Baerbock of the Greens, who are third in polls at 15%.

Current polls, which show a highly fragmented picture as voters increasingly flock to smaller parties, leave room for several coalition scenarios, giving the liberal Free Democrats a potential king-maker role in upcoming coalition talks.

FDP party chief Christian Lindner on Sunday rebuffed demands by the CDU to rule out a so-called traffic light coalition with the SPD and the Greens. "We will not take orders from this (CDU)," he said at a party event.

Merkel's chief of staff had earlier called on all parties to agree quickly on who should succeed her after the election and avoid the kind of protracted coalition talks that followed the last vote four years ago.

The likelihood of long coalition talks after the vote means Merkel will not be leaving office any time soon. She remains chancellor until a majority of Bundestag lawmakers elect a successor, who is then sworn in.