The reporter, 29-year-old Laura Himmelreich, wrote in the weekly Stern that Rainer Brüderle of the co-ruling Free Democrats (FDP), leered at her breasts and told her, "You could also fill out a dirndl," Bavaria's low-cut traditional dress for women, during an informal chat after a party congress in 2012.
He then told her, "Politicians always fall for journalists," according to Himmelreich, in an article that recounted what it called frequent unwanted advances by frisky officials toward female journalists.
She came forward after Brüderle, 67, a former economy minister, was tapped this week as chief candidate for the FDP, junior partner in Chancellor Angela Merkel's government, in September general elections.
Brüderle has declined to comment but top FDP officials flew to his defence.
"This type of reporting a year after a purported incident is deeply unfair," Foreign Minister Guido Westerwelle said.
"This is so transparent and base that it says more about the journalism from Stern than about Mr Brüderle," said FDP deputy Rainer Stinner.
Even the opposition Social Democrats chimed in. "It shows an odd understanding of one's profession to want to have an official discussion with a politician in a hotel bar at midnight," MP Sebastian Edathy told the daily Tageszeitung.
However several editorials described boorish behaviour encountered by women on the job and the wall of silence around the issue, even in a country whose most powerful political figure is a woman and where relations between the sexes usually compare favourably with neighbours such as Italy and France.
Nina Bovensiepen of the daily Süddeutsche Zeitung called the incident "relatively harmless" but said it touched on a serious problem.
"This business is still dominated by men," she wrote. "Occurrences of sexual lewdness and line-crossing are surely more common here."
A senior editor at the online service of Der Spiegel magazine, Patricia Dreyer, said she was "grateful" to Himmelreich.
"Female journalists experience stupid remarks, dumb come-ons, being reduced to their appearance and discrediting based on their gender just as often as millions of other women in other fields, and are just as sick of it," she wrote.
She noted that alcohol-fuelled "briefings" on the sidelines of political events were crucial for reporters seeking behind-the-scenes information.
The Tageszeitung's Anja Maier added: "The malice against Himmelreich, the denigration and the vulgar remarks – these are all sure signs that she did exactly the right thing: to blow open the murky side of the relationship between politics and the media."
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The debate drew a massive response on social media, with the Twitter hashtag #aufschrei (outcry) attracting a flood of complaints about sexism in everyday German life, as well as a backlash against "unfounded" accusations by women.
It also came amid a heated discussion about whether Germany needs quotas to deal with a lack of women in its boardrooms.
Asked about the Stern article, Merkel's spokesman Steffen Seibert declined to comment on the accusations against Brüderle but he said the ground rules for such interactions ought to be clear.
"Of course the chancellor believes in a professional and respectful approach in politics as well as between politicians and journalists," he told a regular media briefing.