Bruce Hyer quit the NDP caucus Monday to sit as an independent, miffed that he was left out of the new leader's shadow cabinet and fed up with what he described as stifling party discipline.
Hyer's messy split came just three hours after Mulcair's new House leader, Nathan Cullen, pronounced a new era in the House of Commons, boasting that New Democrats are "re-energized, reunited and ready" to take on the Harper government.
"We're starting to see some of the fractures that the more autocratic style of this prime minister have not allowed to surface in the past," Cullen said outside the Commons, while promising a "serious, structured and substantive opposition."
Cullen, who ran for the leadership himself, said Monday's resumption of Parliament after a two-week break marks the end of the NDP's long, and sometimes bruising, leadership contest.
"Particularly now that we've settled the shadow cabinet, we've brought this team together," he said.
"There's a tradition with some of the other parties that after a leadership race, their race continues in another form. For us ... that's not what's going to happen."
The words were barely out of his mouth before Hyer proved again that nothing is certain in politics.
The Thunder Bay, Ont., MP said he believes he can better represent his constituents as a lone wolf than by abiding by the "lockstep discipline" required by all parties.
"I didn't come here to be a trained seal," Hyer said in an interview.
Hyer, first elected in 2008, was one of two New Democrat MPs who were punished earlier this year after they voted in favour of scrapping the gun registry, in defiance of the NDP's position. Hyer and neighbouring MP John Rafferty were stripped of their roles in the shadow cabinet of interim leader Nycole Turmel and were banned from asking questions or making statements in the Commons.
Last week, Mulcair named Rafferty his northern Ontario development critic. But he gave no role to Hyer, who took that as a message that the gag order against him was to remain in force.
"When he announced his shadow cabinet, I was clearly being punished and muzzled and that was sad," Hyer said.
"I found it unfortunate that immediately after the (leadership) election that he felt the need to become so controlling so I opted out."
Hyer suggested he was also being punished for not backing the winner. He noted that Rafferty had backed Mulcair.
Rafferty issued a release calling Hyer's defection "a tremendous mistake."
While he's keeping his NDP membership and still maintains Mulcair will make a good prime minister, Hyer took a number of pot shots at the new leader on his way out the door.
He complained that Mulcair — whom he described as "a bright guy, he's a strong guy, he's got a huge ego" — is determined to reinstate the gun registry and "does not seem willing to co-operate with other parties on important issues," such as climate change.
That, too, undermined the official NDP message Monday. Cullen had promised an opposition open to collaboration with all comers.
"Let me be explicit: If the government or any other party is willing to work with us to get something good done for Canadians, then they will have an open ear from New Democrats," Cullen said before Hyer's bombshell.
Hyer is the second New Democrat MP to bolt the caucus since a surge in last May's election gave the party a historic 103 seats. Quebec MP Lise St-Denis, who had been supporting Mulcair's leadership bid, defected to the Liberals in January.
"It certainly demonstrates that life in Mulcairland is not as sweet as some might think," remarked Liberal MP Ralph Goodale.
Mulcair said his decision not to include Hyer in his shadow cabinet was not due to his past voting record on the gun registry or his leadership choice. Rather, he said Hyer was left out after making it clear he wouldn't necessarily toe the party line on issues.
"Bruce had also very clearly communicated to me that he wasn't going to be bound by the decisions of caucus or of the party or anybody else. So it's quite obvious why you can't name someone to the shadow cabinet when they're telling you up front that they're not going to follow the decisions of their colleagues."
Mulcair said caucus solidarity is part of the Canadian parliamentary system, which "might not be the best system in the world but it's the only one that we've got.
"So, Bruce is not able to work within that system. The result is his departure today. It's not an ideal situation. It's not the one that I would have preferred, of course."
The loss tripped up the NDP's carefully choreographed parliamentary reset following a two-week break in the parliamentary schedule.
The Commons debut of Mulcair's freshly minted shadow cabinet proved a low-key affair.
Neither the prime minister nor Liberal Leader Bob Rae were in the House, at least 20 seats on the government benches were empty and catcalls and heckling were at a minimum.
The NDP made a point of having six younger female critics stand and launch questions, including one by Ruth Ellen Brosseau, the accidental MP made famous last year by getting elected in a rural Quebec riding while working as a university bar manager in Ottawa.
But Hyer was the day's story.
Mulcair learned of Hyer's decision to quit the NDP caucus through the media, only a few minutes before it was publicly announced. He said he sent Hyer an immediate message that he was willing to discuss the matter but did not hear back from him.
Mulcair said Hyer also appears to have misinterpreted his commitment to reinstate a gun registry should the NDP form government. He reiterated that it wouldn't be the same as the registry recently abolished by the Harper government but an improved one that protected public safety without antagonizing legitimate gun owners.
Hyer's departure leaves the NDP with 101 seats. There are 165 Conservatives, 35 Liberals, four Bloc, one Green member and two Independents.
Also on HuffPost