In an opening address to the Australian Workers' Union's national conference on Monday, the national secretary said Ms Gillard was "street tough".
"We support her because we want someone leading this nation who's prepared to make the tough calls, the hard decisions, to cop the flack," Mr Howes told the gathering on the Gold Coast.
"Someone who is prepared to stand up to the shock jocks and the billionaires and the media barons."
But he rejected suggestions the AWU had sold out its membership base, which is about 140,000 nationally.
"We didn't sell out our principles or sell out our members' job security," Mr Howes said.
"We walked out at the end knowing that we struck the best possible deal for our members."
The union boss also criticised mining bosses, describing them as "corporate robber barons, in their shiny suits".
Ms Gillard will address the conference, which is being attended by about 500 delegates, on Monday night.