The furore surrounding Victorian construction union boss John Setka has escalated with his deputy Shaun Reardon quitting, saying it was "untenable" for him to continue.
The resignation comes as members of the construction and mining union are being individually lobbied to support Mr Setka, who faces calls for his expulsion from the Labor Party and resignation as Victorian secretary of the CFMMEU.
"Due to the irreconcilable differences between myself and ... John Setka it is with a heavy heart I hereby give my resignation to the branch," Mr Reardon said in an email to colleagues.
He wrote he had not made the decision under pressure.
"Quite the opposite. It is purely a decision made with my family based on personal values I will live and die by," he said.
"The fact is it is untenable for our executive if we can't work in unison on certain issues that I value," Mr Reardon added.
Mr Setka's membership of the Labor Party has been suspended, pending a move to expel him at a national executive meeting on July 5, over accusations he told colleagues anti-family violence campaigner Rosie Batty's advocacy has led to men having fewer rights.
Mr Setka rejects the allegations.
The ACTU and more than a dozen individual unions, including the three biggest in the country, have called on Mr Setka to step aside.
Resisting the pressure, branch officials are writing to 130,000 individual members setting out why the union's leadership is standing by him.
In one of the letters, South Australian CFMMEU secretary Andrew Sutherland says at no stage was Mr Setka trying to disparage Ms Batty or her work to highlight domestic and gender-based violence.
"These are values that the whole labour movement shares and we carry them proudly," he wrote of the values promoted by Ms Batty.
"At no time did I believe John was expressing anything counter to these values."
On Wednesday, the national leadership of the CFMMEU issued a statement in support of Mr Setka.
"The construction and general division strongly condemns the use of untruthful leaks from the (union national executive meeting, which led to the sacking call)," it said.
"At no time at the (national executive) meeting on June 5 did John Setka criticise or denigrate Rosie Batty or indeed any campaign against family violence. John's heartfelt account of his personal life in recent months was received by most in the room in the same genuine spirit in which it was delivered."
Victoria's CFMMEU has vowed to end all financial support to the ALP if Mr Setka is expelled from the party and to cut ties with unions that have "attacked the branch".
The union nationally has provided well over $11 million in donations to Labor over the past two decades.
Liberal frontbencher Peter Dutton said Mr Setka's expulsion was a "foregone conclusion".
"I reckon this is a phoney war - Anthony Albanese is smart enough not to pick a fight with this guy unless he knows the outcome," Mr Dutton told 2GB Radio.
The real challenge for Mr Albanese was to sever ties with the CFMMEU, as Bob Hawke did with the Builders Labourers Federation, he said.
Mr Setka faces court on June 26 when he is expected to plead guilty to two criminal charges, including one of using a carriage service to harass a woman.
