THE Liberal backbencher Mal Washer says he could support Labor's mining profits tax, giving the Gillard government a potentially crucial vote as it seeks to get the controversial measure through Parliament.

Tony Abbott has said the Coalition will oppose the tax and repeal it if it becomes law, but Dr Washer told the Herald: ''I think an appropriately applied mining tax is a very good thing.''

Lifeline ... Malcolm Washer. Credit:Paul Harris

Dr Washer's support could be crucial as Labor faces conflicting demands, with the Greens and some crossbench independents arguing the tax should raise more revenue, but the Tasmanian independent Andrew Wilkie is considering moving amendments sought by smaller miners that could reduce its estimated return of $11 billion over the first three years.

Mr Wilkie's negotiating power on a separate policy was jeopardised yesterday as Labor backbenchers seized on Mr Abbott's ''prediction'' the Coalition would oppose the independent's plan for mandatory pre-commitment on poker machines.