Armin Laschet, leader of Angela Merkel’s centre-right Christian Democrats, has admitted mistakes, after he was accused of plagiarism in a decade-old book.
It came weeks after a similar controversy engulfed his main rival for the chancellorship, Green party candidate Annalena Baerbock.
Mr Laschet’s 2009 book on immigration came under scrutiny after an author highlighted passages strikingly similar to those in his own work.
Karsten Weitzenegger said he had been alerted to the similarity by an anonymous “plagiarism hunter”.
After it came to light, Mr Laschet apologised and said he would request a review of the book to check for further lapses.
“There are obviously mistakes, for which I bear responsibility,” he said.
The setback for Mr Laschet comes amid a dip in his party’s poll ratings, following what was seen as a weak reaction to Germany’s sisastrous floods.
Mr Laschet, the premier of North Rhine-Westphalia, one of the hard-hit regions, blundered by chuckling with colleagues on camera while the country’s president delivered a solemn speech on the flooding.
His party nonetheless remains in a strong position in the polls after Ms Baerbock’s early momentum faded.
The Greens were on a polling high in April but their campaign was weakened by a series of missteps including Ms Baerbock’s own plagiarism scandal.
Ms Baerbock acknowledged that she should have used a list of sources in a book which was published for this year’s election campaign.
She was also criticised for inaccuracies on her CV and for her failure to declare a payment from the party.
Germany goes to the polls on September 26 with Mrs Merkel not seeking a fifth term as chancellor after 16 years in power.
Plagiarism allegations have led to the downfall of a number of government ministers during Mrs Merkel’s tenure.
Most recently, Families Minister Franziska Giffey stepped down in May amid questions over her doctoral dissertation at the Free University of Berlin.
Ursula von der Leyen, the head of the European Commission and previously a German Cabinet minister, was cleared of similar allegations in 2016.
Hannover Medical School, her alma mater, said the passages were a mistake rather than misconduct.