Congress of SA Trade Unions (Cosatu) head Zwelinzima Vavi claimed that DA leader Helen Zille "bugged" him to join her party, the Sunday Times reported.
Vavi told the newspaper that Zille said to him at a football match: "Can you imagine... me and you, the power?"
Vavi claimed it was a preoccupation of Zille's to recruit him to the Democratic Alliance.
"She is very, very power-hungry because she has this mistaken view... that I can be recruited to the opposition ranks.
"She tried to recruit me, in person, to join the DA in the run-up to the local elections."
The two have since bumped heads, with Vavi banning Zille from an e-toll protest march and Zille claiming Cosatu had failed the unemployed.
"That is how she dreams. I can never join her party [whose] very sense of existence is to continue the exploitation of workers."
Vavi said he and the DA were like "water and paraffin" and he believed that Zille would take him on "at a personal level" because he had refused her advances.
Zille's spokesman Pierre-Charl du Preez would not confirm the claims, but dismissed them as "facetious".