The Unite union is urging Liberal Democrats to tear up their membership cards rather than be associated with cuts in public spending.
It has accused the party's leadership of "falling hook, line and sinker" for an agenda of cuts in public services.
Joint leader Derek Simpson said the Conservatives were using the UK's huge budget deficit as "cover".
He also said they were using the Lib Dems for the authority to push the cuts through.
Mr Simpson said: "Under the ConDems, working people will suffer the most, no matter what Nick Clegg says about the cuts being progressive.
"Liberal Democrat members with a social conscience should tear up their membership cards and join Labour to save the shame of being associated with this ruthless right-wing agenda.
"The public did not vote for early and deep public spending cuts, as both Labour and the Lib Dems in the general election campaign opposed severe and immediate austerity measures, warning that the consequence may be a double-dip recession."
Mr Simpson said the Conservative and Liberal Democrat coalition "seem to have no plans to build, only to cut.
"The government want to attack public sector jobs, public sector pensions and the vital public services that ordinary working families rely on."
Mr Simpson also said there had been "unprecedented scaremongering" by Prime Minister David Cameron and Chancellor George Osborne about the state of the economy and the need to save money.