He said he walked away from the offer because the government made it clear they weren't going to respect Canada's international environmental obligations, though he's also being accused of stepping away because the salary was too low.
But in contemporary Canadian politics, many politicians have seen their allegiances change — before his talks with the Tories, Mulcair was a Quebec Liberal. Here are some other prominent politicians who've switched their political colours over time for what they argued were philosophical reasons.
According to several biographies of the current Conservative prime minister, when he was in high school in the 1970s, he ran in Liberal circles.
The prime minister of the time was Pierre Trudeau, and Harper was recruited by one of his classmates to join his high school Liberal club.
But soon after graduation, Harper moved west and as the story goes, lost respect for the elder Trudeau over the implementation of the national energy program, beginning his long road to leader of the Conservative party.
Trudeau also prompted Rae to get involved in politics, where he got his first taste of the life volunteering for the Liberals.
After returning from studying in England, he joined the New Democrats, eventually being elected as an NDP MP before leaving Ottawa to lead the provincial NDP and becoming premier of Ontario.
In 1998, he resigned from the NDP but didn't sever political ties with them until 2002.
That year, he argued in an essay published by the National Post that he no longer supported the party's approach to the Middle East and its opposition to the World Trade Organization.
“This is not a vision of social democracy worthy of support," he wrote at the time.
In 2006, he declared his allegiance to the Liberals, running for its leadership but losing.
He became a Liberal MP in 2008 and the party's interim leader following the 2011 election.
Mulcair named former Conservative cabinet minister Lawrence Cannon as the one who approached him to join the Conservatives.
The duo knew each other from their days with the Quebec Liberals; Cannon represented that party in the Quebec National Assembly for nearly 10 years before Mulcair joined their ranks.
And Cannon backed Sheila Copps when she ran for leadership of the federal Liberals in 2000.
But he then left the party for the private sector, reportedly disillusioned over Jean Chretien's handling of the 1995 Quebec referendum and the subsequent sponsorship scandal.
He went back into municipal politics for a time and then joined the Conservatives.
"The decision of Lawrence Cannon to join the Conservative party will help us build a new, clean federalism in the province," Harper told a 2005 press conference announcing Cannon's candidacy.
He was elected in 2006, serving in two cabinet posts before losing his seat in the 2011 election and being appointed ambassador to France.
The chance to represent Canada abroad also appeared to lure David Emerson across the aisle.
The long-time businessman was wooed by the Liberals to join their party for the 2004 election and he was elected in a Vancouver riding that year, going on to become industry minister.
In the 2006 election he ran for the Liberals again, repeatedly attacking the Tories, and won his seat. That campaign saw the Harper Conservatives eke out a minority government victory.
When Harper and his team showed up at Rideau Hall to be sworn in, Emerson was with them and was named international trade minister.
The Toronto Star had reported that while in the Liberal cabinet, Emerson had objected to a softwood lumber deal the government was close to disclosing. Emerson told reporters he made the partisan switch to serve his constituents better.
"I am pursuing the very agenda that I got involved to pursue when I was in the Liberal party supporting Paul Martin. I'm continuing to pursue it," he said in 2006.
He did not stand for re-election in 2008.
The Nova Scotia MP was first elected in 1997 as a Progressive Conservative and in 2003 ran for the leadership of that party, losing to Peter MacKay.
Though later that year he voted in favour of the PC's merging with the Canadian Alliance to form the new Conservative party, only days after the merger he announced he was going to sit as a Liberal.
In later interviews, he said he was told by those in the Canadian Alliance, including Stephen Harper, that the fact he was gay wouldn't hold back his political career within the new party.
But he said he was also told the Conservatives would continue to champion issues that were important with its socially conservative base.
"I could not run for a party that I did not want to win the election," Brison said in a 2006 interview.
The Toronto-area Tory sent a shock through political circles when she showed up alongside Liberal Leader Justin Trudeau at a news conference earlier this year and announced she was joining his team.
Adams had long ties to the Conservatives dating back to the Mulroney days, and had been elected for the party as an MP in 2011, winning a coveted suburban riding away from the Liberals.
Eventually, she became romantically linked with Dimitri Soudas, Harper's former director of communications who went on to become executive director of the party.
He lost that position after being accused of meddling in her nomination campaign for the 2015 election, which eventually led to both of them being excommunicated from the party.
When Adams announced her decision to join the Liberals, she said it was because she no longer supported the Conservatives' policy approach, specifically their income splitting policy.
"I cannot support mean-spirited measures that benefit only the richest few,” she said.
She has yet to be formally nominated as a Liberal candidate for the upcoming election.