The controversy over spending by senators Mike Duffy, Patrick Brazeau, Pamela Wallin and Mac Harb gave Mulcair the chance to stand out among MPs — and among the three party leaders — for his pointed, careful questions. Harper, forced on the defensive, faltered and was forced to retreat on his support for his former chief of staff, Nigel Wright, as well as on his assertion that it was Wright alone who knew about the deal to cover Duffy's $90,000 in expenses.
Keith Beardsley, a former deputy chief of staff to Harper, says the Senate scandal was pivotal for Harper.
"It's the first time the government has got themselves into pretty heavy criticism and one I suppose where the population is really paying attention," Beardsley said in an interview with CBC News.
Harper also faced increasing grumbling among his MPs, with several complaining in the House about the Prime Minister's Office limiting their ability to speak in the few minutes allotted to backbenchers, and Edmonton MP Brent Rathgeber quitting caucus.
Even some cabinet ministers disagreed publicly. Employment Minister Jason Kenney and Justice Minister Peter MacKay defended Wright after Harper said he'd fired him, and Kenney spoke out against Toronto Mayor Rob Ford's crack use, despite Finance Minister Jim Flaherty's public support for Ford.
"You can see the backbenchers starting to stir a little bit," Beardsley said, adding there could be more of that to come in 2014 as MPs decide whether they're going to run again in 2015.
"There will be a number that, for all sorts of reasons, decide they don't want to run again, so they're going to have a certain degree of freedom and they're going to want to leave their mark … they don't give a damn what PMO says or anyone else says," he said.
The Senate scandal gave Mulcair the chance to show off his skills in question period, asking sharp questions and highlighting Harper's refusal to answer beyond a few talking points.
Gerry Caplan, a former NDP strategist, said Mulcair has been embraced by the media and even conservative commentators for his question period ability.
Caplan calls it "the splendid and completely irrelevant job he's done in the House in pinning Harper to the wall."
"I say irrelevant because Harper hasn't changed his tune and it doesn't seem politically to be helping the NDP," he said.
The Liberals are ahead of the NDP and even the Conservatives in recent polls, though, as Mulcair pointed out in a year-end press conference, the NDP's previous ceiling in polls is now its floor.
Steve MacKinnon, former Liberal Party national director, agreed that Mulcair kept Harper on his toes in question period.
"I think that his work in the House of Commons has been more or less effective, perhaps not in elevating his own stature, but certainly in keeping this scandal on the front pages and on Canadians' minds. He has at times made the prime minister look very, very bad. And speaking as a Liberal, any time the prime minister looks bad, that's a good day," he said.
Canada-EU trade a bright spot
The tentative trade deal Canada is finalizing with the European Union was the one bright spot for Harper in 2013, although it isn't a done deal.
"There's a lot of heavy lifting to be done, both with the provinces and in terms of settling on final language," said MacKinnon, who handles financial transaction files in his role at Hill & Knowlton Strategies.
"Obviously all judgment on any deal has to be suspended until we see that [final text]. But nonetheless, I think a major trade victory there … one that's pretty good for the country."
While Harper's trade policy drew praise, Liberal Leader Justin Trudeau, who won the party's leadership race last April, has been criticized for lacking policy ideas. Trudeau says he's consulting with experts and with Canadians.
Caplan said those explanations usually mean a politician doesn't have any policies.
"That's been true for many, many generations of politics," he said.
"Can he get away with it? We don't know. Sometimes you can. Sometimes you bluff it out.… [But] it's very hard to do because the pressure to say something of substance gets pretty strong."
Trudeau exceeded expectations
MacKinnon said the on-the-ground organizing done by Trudeau's team is under-appreciated. Both Beardsley and Caplan agree Trudeau has done better than expected.
"He still has a way to go in question period. He's not quite there in that," Beardsley said.
"If you look at his first few days versus now, he's much more polished than he was before, so I think he's done well and he's shown that he's got some staying power.
Caplan said he fears Mulcair isn't taking Trudeau as seriously as he should.
Trudeau "has succeeded far beyond what anyone ever thought. I don't know anyone who thought the Liberal Party could come back as far as it appears to have done. Much of it I attribute to him and this astonishing appeal that he has," Caplan said.
"I think [Mulcair] hopes it's a passing ephemeral phenom, which is something that I'm not sure that it is."