Michael Ignatieff shuffled his shadow cabinet Tuesday, arranging his most aggressive and senior MPs into roles aimed at making life uncomfortable for the Harper government.

The Liberal Leader called on his most seasoned MPs to fill the big critic jobs, including finance, defence and Opposition House Leader.

“We need to send a signal to these guys. They want to play nice, we’ll play nice. They want to play tough they should watch who they wrangle with,” a senior Ignatieff official told The Globe. “The Liberals aren’t going to lie down.”

The shuffle builds on the new confidence of the Liberals as a result of a summer caucus meeting last week in Nova Scotia, in which the party appeared unified. It followed Mr. Ignatieff’s summer bus tour, which has been regarded as a success.

Nova Scotia MP Scott Brison takes over as finance critic from John McCallum while New Brunswick MP Dominic LeBlanc replaces Ujjal Dosanjh on the national defence file.

“I think we put very political people in both defence and finance,” the Ignatieff official said.

It will be Mr. LeBlanc’s job to hold Defence Minister Peter MacKay’s feet to the fire over the Harper government’s procurement policies, especially the $16-billion purchase of the 65 next-generation fighter jets.

Central to the Liberal Leader’s moves Tuesday is David McGuinty, who is taking over the key role of Opposition House leader. Mr. McGuinty replaces long-time procedural captain Ralph Goodale, who becomes deputy leader to Mr. Ignatieff.

In his new role, the Ottawa South MP and brother of Ontario Premier Dalton McGuinty – who is pugnacious and a known for his heckling skills in the Commons – will be facing off against the new Government House Leader John Baird.

Mr. Baird, who also represents an Ottawa riding, is no shrinking violet either. His promotion this summer suggested to the opposition that the Conservatives were going to play rougher in the Commons.

“We have been criticized in the past for being passive or being shy or being too timid,” the official said. “Well, if David’s appointment signals something different than terrific.”

It will be Mr. Goodale’s job, meanwhile, to handle Question Period when Mr. Ignatieff is on the road. In the past, that job was often left to Mr. Ignatieff’s leadership rival, Bob Rae, who remains foreign affairs critic.

Mr. Ignatieff is expected to be away a lot from the House of Commons. He announced at his summer caucus meeting last week that he will be holding a series of “Open Mike” town hall meetings in every region of the country.

Another key position is that of health critic, and Mr. Ignatieff tapped Mr. Dosanjh for that role. The British Columbia MP served as Paul Martin’s health minister during the 2004 negotiations with the provinces that resulted in a $41-billion, 10-year deal.

“We’re making health a big issue. Part of our spiel is who are you going to trust to go into the next negotiations and who better to trust than the guy who did it last time?” the Liberal official said.

Some MPs have even been given a second chance. Montreal’s Denis Coderre, who resigned in a huff as Mr. Ignatieff’s Quebec lieutenant, is back in the senior ranks as natural resources critic.