US TRADE unions will freeze their contributions to federal candidates “until further notice,” federation AFL-CIO warned politicians on Wednesday.

Its intention is to channel funds into the intensifying battle to stop or at least help shape a proposed free-trade agreement being negotiated by the US and other nations that border the Pacific Ocean.

The unions are battling legislation going through Congress to fast-track negotiations over the proposed Trans-Pacific Partnership through the House and Senate.

Such a short cut has been used before, notably in the 1993 North American Free Trade Agreement affecting the US, Canada and Mexico.

Earlier US trade deals “form a mountain of broken promises made to workers,” the union federation complained.

Non-federation affiliate the Teamsters also challenged US negotiators on the trade pact to press for a crackdown on Mexican cross-border lorry driving as part of the emerging deal.

White House spokesman Josh Earnest was asked whether the unions’ stance would make it harder for the Obama administration to find Democratic support in Congress for fast-track authority.

“We understand that there are some groups that have traditionally been aligned with the Democratic Party that are very sceptical of any sort of trade deal,” he said.

“But, you know, the fact is the president has made a firm commitment to both Democrats and Republicans that any sort of trade agreement that he signs onto will be one that he firmly believes is clearly in the best interests of American businesses and American middle-class families. And that is not going to change.”