Liberty director Shami Chakrabarti is threatening to sue a Cabinet minister she says "set out to smear" her dealings with ex-Tory MP David Davis.
Ms Chakrabarti said that she would sue Culture Secretary Andy Burnham if "you continue down the path of innuendo and attempted character assassination".
It follows an article in which he said he found her alliance with capital punishment backer Mr Davis "curious".
A spokesman said Mr Burnham had not meant any offence.
Mr Burnham's comments came in an interview with New Labour magazine Progress.
He expressed dismay at civil liberties campaigners and other people who had been "seduced by Tory talk of how liberal they are".
He said he found it "very curious in the man who was, and still is I believe, an exponent of capital punishment, having late-night, hand-wringing, heart-melting phone calls with Shami Chakrabarti".
Oh, I do so hope that Chakrabarti - whose letter to Burnham can be read in full at Liberty's website - carries out her threat. I mean, Labour cabinet minister vs. supremely irritating - and utterly hypocritical - liberal hand-wringer: the only sad thing is that they can't both lose!