The historic link between Labour and the unions could suffer a blow if the firefighters' union votes to cut its financial contribution.
Firefighters have been angered by their pay dispute
The Fire Brigades Union will decide on Wednesday whether to cut its £50,000 annual contribution to £20,000.
It could also vote to disaffiliate from Labour altogether.
Meanwhile, the Communication Workers' Union says it will suspend its £300,000 donation if the government fails to commit to a publicly owned Post Office.
Both unions are holding their annual conferences.
In Bournemouth the CWU backed an emergency motion demanding Labour includes a pledge to keep the Post Office in public hands in its next election manifesto.
Without that commitment future funding will be suspended.
General Secretary Billy Hayes said: "I am confident that the leadership of
the Labour Party and the government will appreciate the need to reassure the
public that postal services will improve and develop.
"The only concrete way this can be demonstrated is a formal commitment for it
to remain in public hands.
Andy Gilchrist is general secretary of the Fire Brigades Union
"I am sure we can gain the guarantee that any hair-brained share scheme from Royal Mail managers will be shown to be an idle daydream."
Meanwhile, the FBU in Southport faces calls to disaffiliate from Labour altogether from a number of union branches still angry in the wake of an acrimonious pay dispute that ended in strike action last year.
The union's executive's recommendation of cutting funding will be put to a vote.
If the executive's recommendation is not carried the conference will vote on an amendment to disaffiliate from the Labour Party altogether.