NDP Leader Thomas Mulcair told his caucus today that the party is beginning a "new chapter" in its history as he vowed to battle Prime Minister Stephen Harper over tomorrow's budget and defeat the Conservative government in the next election.

"This week, we start a new chapter and it's going to be an exciting one. It's a chapter that will end with the defeat of Stephen Harper's Conservatives and the election of Canada's first New Democratic government," Mulcair said during a public speech to the NDP caucus on Parliament Hill.

Mulcair predicted that the budget will benefit interest groups that have the ear of the government while the middle class will be left out in the cold.

"In 35 years of service, as a cabinet minister and a member of Jack Layton's team, I have always fought to put the public interest first, and tomorrow, if Canada sees no action to boost job creation, if pensions and health care are cut and Canadians are left footing the bill, then Mr. Harper, you will will have a fight on your hands," he said.

The newly elected Official Opposition leader said he hopes the budget will provide relief to the middle class, invest in job creation and protect essential services, but that he's not getting his hopes up based on Harper's track record.

New NDP leader reflect on year

Mulcair went on to describe the risks he says Harper poses to Canadians, including cutting their old age pensions in order to pay for prisons and the F-35 fighter jets, cutting health care, and cutting the public service.

"We will not let them get away with it without a fight," Mulcair said.

Mulcair also reflected on the eventful year the NDP has had. Over the last 12 months, the party:

Went through an election campaign that culminated with a historic victory.

Took over from the Liberals as Official Opposition.

Leader Jack Layton announced he had cancer for the second time and died a short time later.

Nycole Turmel assumed the job of interim leader.

The party launched a lengthy leadership campaign that wrapped up on Saturday with Mulcair's election.

He told his caucus that it was a difficult and painful year at times, but that the NDP came through it stronger than ever before.

Mulcair said it was a moving experience on Monday when he sat in the desk chair in the Official Opposition leader's office once occupied by Layton.

"Sitting in that chair hardened my resolve to carry on the task at hand: to protect Canadians from Conservative cuts, to take on this Conservative government, to reach out with all of you to those who've been abandoned by Stephen Harper," he said.

Finance Minister Jim Flaherty will table the budget at 4 p.m. ET Thursday.

It's one of the most anticipated budgets in years because of the spending cuts it's expected to contain. The public service is bracing for thousands of job losses, and the government is also expected to outline changes to the Old Age Security program and environmental assessment reviews.