Cosatu in the Western Cape is on a collision course with the ANC provincial chairperson Mncebisi Skwatsha, who they do not want as a premier candidate for the province.
Skwatsha tops the ANC's candidates list in the province.
The federation is said to favour current premier Lynne Brown - who is number two on the list.
Brown was inaugurated as premier last August after the ANC's national executive committee sacked Ebrahim Rasool following a lengthy battle between Rasool and Skwatsha.
Cosatu is expected to endorse her when it announces its preferred candidate at a shop stewards' council meeting in Cape Town next week.
Cosatu provincial secretary Tony Ehrenreich said Cosatu "works really well with Brown" and she has done a lot for senior citizens.
Brown is to announce today that the government will fund free trains once a month for pensioners. Until now, Cosatu and Sanlam have been funding the monthly trips.
The labour federation's decision will shake up Western Cape politics because Brown was only supposed to be a "stop-gap premier" until Skwatsha took over.
However, Skwatsha fell into disfavour with some ANC members who blamed him for the ANC's failure to register its candidates before recent by-elections.
The behaviour by the Skwatsha faction led to Luthuli House intervening and taking over the running of the ANC administration in the province.