Unions fear conservative state governments are pushing to ''silence'' them nationally after Victoria, Queensland and Western Australia joined NSW in defending its political donations laws against a landmark High Court challenge.
A hearing is due to begin on Tuesday in the case that has been launched by the peak body Unions NSW and five unions in a bid to have the NSW government's donations laws declared unconstitutional.
The laws ban corporations and associations from making political donations in state and local government elections in NSW.
They also restrict how much the Labor Party and its affiliated unions can spend on advertising during an election campaign by counting expenditure by unions against the total amount the party is allowed to spend.
The attorneys-general of Queensland, Victoria, Western Australia and the Commonwealth have ''intervened'' in the case to support the NSW defence.
Labor and the union movement have accused NSW Premier Barry O'Farrell of using the laws, which he has cast as an attempt to reduce the influence of donations in NSW politics, to attack their ability to contest elections.
The unions will argue in the High Court that the laws restrict the freedom of political communication implied in the NSW and federal constitutions.
In its submission NSW says there is no implied freedom of political expression in the state constitution and the federal constitution is not relevant because ''the conduct of state elections is confined to the states''.
The Commonwealth submission argues that while the laws ''effectively'' restrict freedom of political communication ''they do so for ends that are plainly legitimate or permissible''.
The secretary of Unions NSW, Mark Lennon, said the decision to intervene ''shows there is a clear agenda to silence the political voice of working people, not just in NSW but across the country''.
''Working people have always pooled resources to express themselves politically,'' he said. ''The point of our case is to make sure that can continue and we have robust political representation in this country''.