Australia's upper house has begun debating a bill that is likely to trigger a double dissolution election.
Governor General Peter Cosgrove has recalled both houses of parliament to Canberra for an additional sitting, at Prime Minister Malcolm Turnbull's request.
The recall is to allow debate of a bill to re-establish a commission overseeing the building industry.
The upper house, known as the Senate, has already rejected the bill once.
It is expected to be rejected for a second time, giving the prime minister a trigger to call the election.
The double dissolution is a mechanism in the Australian constitution that allows the government to call for an election if a piece of legislation is blocked twice in the upper house.
The mostly likely trigger for an early election is the Australian Building and Construction Commission (ABCC) bill, which seeks to re-establish a watchdog to monitor union activity in the construction industry.
A second piece of legislation, the registered organisations bill, seeks to hold unions to higher standards of transparency. It could also provide the double dissolution trigger.
The government needs support from six of eight independent and minor party senators, known as crossbenchers, to get the ABCC bill passed. Labor and the Greens do not support the bill.
But several key crossbenchers have indicated they are likely to oppose the legislation.
Senator Ricky Muir of the Motoring Enthusiasts Party told The Australian newspaper that the government was not serious about passing the bill and wanted "a double-dissolution at all costs".
The opposition repeatedly attacked Mr Turnbull's decision to recall the parliament using an obscure provision in the constitution that has not been used for 40 years.
Labor Senator Stephen Conroy accused Governor General Peter Cosgrove of demeaning his office by calling back parliament to debate the bills.
Senator Conroy said the decision to recall both houses of parliament was a "tawdry political stunt".
"Since 1961 … the parliament has only been prorogued four times, under extraordinary circumstances, and never, never to set the scene for an election," Senator Conroy said.
"A strong governor general would never have agreed to this."
Mr Turnbull said that Senator Conroy had "disgraced" himself.
"I look forward to the leader of the opposition publicly dissociating himself from those appalling remarks," Mr Turnbull said.
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