Six crossbench senators appear set to torpedo Malcolm Turnbull's tax cuts for big businesses, one of the centrepieces of his economic plan.
After months of public argument, the company tax cuts are expected to go to a vote in the Senate later this week before the parliament rises for the winter break.
The prime minister is still arguing the case but he has only convinced four of the 10 crossbench senators to back him - he needs eight.
"The case for reducing company tax is a case for Australian workers," Mr Turnbull told parliament on Monday.
He wants the upper house to cut the corporate tax rate to 25 per cent, but so far the coalition has only secured a cut for businesses with a turnover of up to $50 million.
Pauline Hanson's One Nation remains opposed to cutting taxes for multinational companies, especially while she fights the Longman by-election in Queensland.
Senator Hanson wants a crackdown on multinational tax avoidance and an end to tax deductions for firms using overseas call centres.
"The government has not given me any assurance that they intend to go after multinationals to get them to pay their taxes in this country," Senator Hanson told Seven Network.
Mr Turnbull said Australia has done more than any other country to crack down on tax avoidance, with more than $7 billion already recovered.
"We've been relentless on this," he told reporters in Canberra.
Without the votes of the One Nation senators, the legislation looks doomed - even if the policy isn't.
"That's the policy we're taking to the next election," Deputy Prime Minister Michael McCormack told ABC radio.
The government fell short two votes when it tried to get the remainder of the tax plan passed just before Easter.
Since then Senator Hanson reneged on a previous deal with the government to support the cuts.
The two Centre Alliance senators haven't budged in months, neither has independent Tim Storer, while Derryn Hinch wants the turnover threshold set at $500 million.
Labor's Senate leader Penny Wong tried to bring the vote forward on Monday in a bid to force the crossbench to reveal which way they will vote.
"It's time for senators here to put up or shut up. Are you being straight with the Australian people?" Senator Wong told parliament.
But Senator Hanson refused to support debate being brought forward, although she insists her opposition to company tax cuts is unchanged.
"You are nothing but bullies in here. I think you need to wake up to yourselves and have a good look in the mirror," Senator Hanson said.