The former German chancellor Gerhard Schröder has been out of office for more than 13 years, but he is still capable of causing mischief at the highest levels of his beleaguered Social Democrat party (SPD).

In an interview with the German news magazine Der Spiegel published on Friday, Schröder criticised the SPD leader, Andrea Nahles, saying she had made “amateur mistakes” and did not have the key qualifications he considered necessary for serving as chancellor.

Instead, Schröder praised the former SPD chief Sigmar Gabriel, calling him the party’s “most talented” politician and suggesting he would be a better contender for the job.

Germany's Social Democrats elect first female leader Read more

Schröder is known for being outspoken, and as an ex-chancellor his words carry weight in Berlin. But his comments struck a nerve within the SPD, which governs as the junior partner in a grand coalition with the conservative CDU party and is struggling with historically low support in the polls.

With several important elections on the horizon this year – both for the European parliament and in three former East German states – the party’s effort to rebrand itself and stop bleeding support has taken on greater urgency.

The SPD won only 20.5% of the vote in the September 2017 general election, the party’s worst result since the second world war, prompting some activists to caution against entering into another coalition with the CDU.

Nahles, a former leader of the Young Socialists who was once seen as a rising star on the party’s leftwing, was elected leader in April, tasked with reversing her party’s decline by establishing a more combative and confrontational approach towards the CDU.

However, support for the SPD has slumped even further and it is polling at about 15%, below the Greens and on a par with the far-right Alternative für Deutschland (AfD). In some states, such as Bavaria, the figure has dropped to single digits.

Schröder is no stranger to controversy in his post-chancellorship years. He has taken up positions with Russian oil companies and enjoys a cosy relationship with the Russian president, Vladimir Putin.

Many top SPD politicians have come to Nahles’s defence. The foreign minister, Heiko Maas, said that “of course” he trusted Nahles’s leadership, and the party’s vice-chairman in parliament, Karl Lauterbach, said Schröder’s critical words were unfair to Nahles.

Ralf Stegner, one of Nahles’s deputies, said such infighting would only hurt the party. “Does anyone believe it will be of any use to their own party when retired politicians rudely comment on their successors?” he tweeted. “That only benefits the political competition.”

But Schröder’s views have also garnered some support from party members who were already critical of Nahles. Boris Pistorius, the interior minister of Lower Saxony and a member of SPD leadership, told Die Welt that the SPD’s sidelining of heavyweights such as Gabriel and Martin Schulz risked alienating voters.

Asked whether Nahles and the finance minister, Olaf Scholz, had done a poor job of leading the party, Pistorius demurred. “They certainly haven’t chosen the easiest time to lead and renew the SPD,” he said. “At the moment that is a thankless and difficult task.”