In recent days the government has been forced to admit what most informed commentators already knew: the tax will raise little money in its first year. 

This year the much-vaunted Minerals Resource Rent Tax (MRRT) will scratch together barely $126 million.

And it has been reported  the mining giants Rio Tinto and BHP Billiton have already accumulated $1.7 billion in tax credits to offset against future liabilities.

Combine this with the government’s foolish agreement to make present and future state mining royalties fully deductible against the federal tax and it is clear the MRRT cannot possibly be the cash cow that Labor said it would be.

This is a huge problem for the government, because much of the money the tax was supposed to raise has already technically been spent – allocated to new initiatives including disability support reform.

So, as the Gillard government drifts towards an election where a major defeat could put Labor in opposition for multiple terms, it must decide what to do about its MRRT budget black hole.

Backing away from its big-ticket reform proposals would be politically suicidal, as that would mean disappointing the expectant electorate as well as admitting its flagship tax was botched.

The mining giants, flush with billions of dollars in profits from Australian coal and iron ore, have warned the government to leave them alone, taking out full-page newspaper advertisements this week, arguing they were already paying a big enough share of taxes and royalties.

Tony Abbott’s Coalition stands to inherit the headache if it wins power in September, but it can afford to let the government suffer alone, waiting until after the election to declare that Labor’s budget and tax mistakes necessitate a recasting of priorities.

Voters will ultimately pay, no matter which side wins office.