Angela Merkel has sought to scotch rumours of a rift with her party’s new leader, Annegret Kramp-Karrenbauer, dismissing reports that she thinks her successor is not up to the job.

The German chancellor defended Kramp-Karrenbauer after the new leader was criticised for calling for a clampdown on opinionated YouTube clips ahead of elections.

The Christian Democratic Union (CDU), which has been led by Kramp-Karrenbauer since Merkel gave up the role in December, was caught off-guard earlier this month by a blistering, nearly hour-long attack on the party and its coalition partner, the Social Democratic party, by Rezo, a 26-year-old YouTube star. The video has been viewed more than 13m times.

Both parties delivered poor results in the European parliamentary elections, with the CDU receiving its lowest-ever result in a national poll. Discussions of a “Rezo effekt” are rife in political circles.

Merkel said a report in Bloomberg quoting two unnamed officials who said Kramp-Karrenbauer had lost her predecessor’s support was “rubbish”.

“I have never concerned myself intensely with rubbish in my many years of political engagement, which is why I’m not going to comment on this further,” she said in Brussels, where she was attending an EU summit.

Kramp-Karrenbauer has faced a storm of protest, particularly on social media, since she called earlier this week for a debate on whether internet commentators and influencers should be bound by the same rules as other parts of the media ahead of elections.

“What would the reaction in this country have been had 70 newspapers, two days before the election, declared: … ‘Do not vote CDU and SPD’?” Kramp-Karrenbauer said after Sunday’s poll. “The question needs to be asked ... what are the rules in the analogue sphere, and which rules apply to the digital sphere?”

Critics accused her of trying to quash freedom of speech, and about 50,000 people have signed an internet petition under the slogan “We are YouTubers and we have an opinion”.

Merkel defended Kramp-Karrenbauer, saying freedom of speech was considered a basic principle in the CDU. “Everyone that I know in the CDU, men and women, are committed to freedom of speech as an elementary principle,” she said.

Kramp-Karrenbauer won the job of CDU leader after receiving the backing of Merkel, who is widely believed to see her as a potential successor as chancellor if the CDU wins the next election. Merkel has said she will not stand for a fifth term.

But so far Kramp-Karrenbauer has failed to shine. She is 20 points behind Merkel in the popularity ratings, and the CDU’s poor showing at the European parliamentary elections – it dropped 7.5 percentage points, securing 22.6% – is a far cry from Kramp-Karrenbauer’s aim to improve the party’s fortunes and push it up to 40%.

The party faces further electoral tests in three state elections in September.

Kramp-Karrenbauer was also criticised for her reaction to the Rezo video, in which he declared he wanted to “destroy” the CDU with his own facts and figures, before listing what he called its failings on everything from climate issues to social equality. She said: “I asked myself, why not also make us responsible for the seven plagues back then in Egypt.”

Some called her remarks flippant, and said they demonstrated a lack of understanding over the frustration among voters, with whom the YouTuber seems to have struck a chord.

Admitting the CDU had been slow to react to Rezo’s video, the most watched of its kind in Germany, Paul Ziemiak, the party’s general secretary, said: “Our resources in online communication are far too scarce for us to be able to get in touch with young people.”

But within the party itself, the criticism pointed mostly to Kramp-Karrenbauer. Norbert Röttgen, a leading CDU politician, said: “The CDU leadership was not in a position to give an adult response to it.” He accused the leadership of a “dramatic communication deficit”.

Sometimes referred to as “mini-Merkel”, Kramp-Karrenbauer had already courted controversy earlier this year with a joke about gender-neutral bathrooms during the carnival season, saying they were “for the men who don’t know if they are still allowed to stand or already have to sit down when they pee”.