Left-wing union United Voice is poised to call on controversial West Australian Labor candidate Joe Bullock to reconsider his Senate position.

The ABC has learned the union will make the call to try and pressure Mr Bullock into relinquishing his number one spot on Labor’s Senate ticket.

Labor’s primary vote collapsed to less than 22 per cent in last Saturday’s Senate election re-run in WA.

That was bad news for Labor’s number two candidate Senator Louise Pratt, who is fighting for her political life against Liberal Linda Reynolds.

Senator Pratt was consigned to Labor’s second spot due in a factional deal brokered by United Voice and the right.

The deal saw United Voice’s candidate, the left faction’s Sue Lines, installed immediately into the Senate, through the vacancy created by the departure of former minister Chris Evans in 2013.

In exchange, Mr Bullock, the right’s candidate, won the top spot on the Senate ticket for the September general election.

But a speech Mr Bullock gave in November last year, which was only made public on the eve of the Senate re-run, has prompted United Voice to call on the Senator-elect to stand down.

Mr Bullock has since apologised for the address, in which he praised Liberal Prime Minister Tony Abbott, criticised some Labor members as “mad”, and took aim at some of the party’s more progressive policies.

And he emailed members to say sorry for commenting about Senator Pratt’s sexuality and her work in supporting same-sex marriage.

The secretary of United Voice WA, Carolyn Smith, is due to make a statement to the media in Perth at 3:00pm AEST.

ABC election analyst Antony Green says while the union can call on Mr Bullock to stand down, they cannot force him to leave.

“Nobody can force him to resign, nobody can resign on his behalf, the seat is his,” he said.