The freshly minted NDP leader took part in his first formal Parliament Hill caucus meeting after winning the leadership on the weekend.
He delivered a pep-rally speech to his MPs.
"Here we are, once again more united than ever before and ready to roll up our sleeves and stand up and face off against Stephen Harper's right-wing policies," he said to waves of applause.
He said the business of defeating Harper starts now and will end with a New Democrat government in the next election.
"The hard work starts now," he said.
Mulcair portrayed the prime minister as an ideologue whose policies will help the well-to-do and hurt ordinary people.
"In this Conservative budget, it seems that once again that the well-connected interests with the ear of this government will be the big winners and the middle class will be left out in the cold," he said.
"If the Conservatives abandon their promise to create jobs and instead of that attack our health-care system and our pension system, well we'll be there to stop you, Mr. Harper."
He said budgets involve making choices, but he doubted Harper would make the right ones.
"Mr. Harper has an opportunity to give some respite to the middle class and to invest in job creation. He can protect essential services that Canadians rely on. I hope that that's what he is going to do, but we shouldn't be dreaming in Technicolor, especially with him."
He carried that theme into question period in the Commons, saying cuts to services will hurt ordinary Canadians.
"Is it that the Conservative government doesn't understand, or is it that they just don't care?" he said.
Mulcair was cheered long and loud through his caucus speech, with the decibels rising sharply as he welcomed Craig Scott, who won a Toronto byelection earlier this month to fill the vacancy left when Jack Layton died.
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