Mulcair, a combative former Quebec Liberal cabinet minister, won the NDP leadership on a fourth ballot, besting longtime party strategist Brian Topp in a contest that severely strained the party's self-styled tolerance.
A perceived centrist who was once wooed by Harper's Tories, Mulcair overcame loud complaints that he would abandon social democratic principles in the pursuit of power — a federal pursuit that New Democrats can now truly taste for the first time in their 50-year history.
"Now we have to take the next step," Mulcair told the much-diminished crowd still at the Metro Toronto Convention Centre 12 hours after the day's voting began.
"Our future is limitless if we get our priorities right."
In the end, a party now dominated by its come-lately orange wave in Quebec went with its star candidate in that province to replace Jack Layton, whose sudden death from cancer last August staggered New Democrats just weeks after their spring electoral breakthrough.
Mulcair claimed 57.2 per cent of the vote in the final, head-to-head showdown with Topp on Saturday's fourth and final ballot.
He raised Topp's fist in his own onstage after embracing all the leadership candidates as well as Layton's widow, Olivia Chow in a much-needed show of unity for the party.
"We understand our only priority is to get together," Mulcair told CBC television after his victory speech. "My only adversary since the beginning of this is the people who sit across from me in the House of Commons."
He paid pointed tribute to Layton, who lifted New Democrats to official Opposition status before succumbing to cancer last summer.
Layton's message, said Mulcair, was "giving people a reason to believe that you can vote for change that you want — and actually get it."
Mulcair assumes the role of leader of Her Majesty's loyal Opposition when the House of Commons resumes Monday after a one-week break.
The experienced legislative sparring partner was the candidate most New Democrats conceded was best prepared to leap into the ring swinging. He'll be put immediately to the test when the Conservatives bring down their first budget as a majority government this Thursday.
"Thomas is fearless, Thomas is organized," NDP MP Charlie Angus said Saturday after his first choice, Paul Dewar, dropped out following the first ballot. "He's one of the strongest MPs we've seen in the House of Commons and he's certainly a match for Stephen Harper."
The Conservatives wasted no time responding, issuing an excoriating missive that called Mulcair "an opportunist whose high tax agenda, blind ambition and divisive personality would put Canadian families and jobs at risk."
Prime Minister Stephen Harper was much more brief, and considerably kinder, when asked during a stop on his current trade mission to Asia what kind of an opponent he expected Mulcair will make.
"We will see," is all he said, then added "Obviously I want to congratulate Mr. Mulcair in his victory. I look forward to interacting with him in the years to come."
Brian Topp, a close Layton confidant and the first candidate to enter the race just three weeks after his death, fell 8,542 votes short on the final ballot against Mulcair.
He and others vowed to work with the new leader in a united front against the Conservatives.
Vancouver MP Libby Davies, a Topp endorser who has locked horns with Mulcair in the past over the Middle East, said she'll have no trouble rallying behind him now.
"I've worked with Mulcair ever since he got elected so we'll be united at the end of the day," she said.
"That's not in doubt. We know that our bigger challenge is taking on Harper and showing that there's an alternative to the Conservative agenda."
Even Ed Broadbent, the former party leader who had endorsed Topp and launched an eleventh-hour broadside against Mulcair's temperamental suitability to lead, joined Mulcair on stage and gave him a somewhat grudging handshake.
Dark-horse contender Nathan Cullen, written off at the start of the race last fall after proposing co-operation with the Liberals, made it to the final three and cemented his role as a rising star in the party.
Cullen said his strong showing in the contest proves there's an appetite for co-operating with Liberals.
"Change is in the wind, my friend," he said. "I think anybody who (thought) New Democrats aren't open to the ideas of change was obviously mistaken."
Peggy Nash was eliminated following Saturday's second ballot, while Paul Dewar, Martin Singh and Niki Ashton all dropped out after the morning's initial vote.
Mulcair, Cullen, Nash and Dewar are among the NDP's best parliamentary performers and their long absences on the leadership campaign trail have not helped the official Opposition consolidate its role.
Their return next week should reinvigorate a Commons already boiling with political controversy over allegations of election fraud and the prospect of a ground-shifting federal budget.
The NDP's weekend leadership showcase, however, was drained of much of its excitement and vigour Saturday by a series of technical delays with the online voting system — although the source of the delay did add a minor element of intrigue.
Party president Rebecca Blaikie confirmed two IP addresses had been isolated as the source of cyber-attacks that appeared designed to slow entry into the system, effectively gumming up the works but not impairing the vote.
"Whoever this is or whatever it came from, their goal was simply to make it a pain to get into our site, to make it harder for people to vote, to block it up with a lot of traffic," Blaikie said.
Whatever the cause, Mulcair's victory wasn't confirmed until late Saturday evening — timing the party had scrupulously planned to avoid.
That wasn't the only come-down for New Democrats.
The day's biggest ballot topped out at 65,108 voters, a less than 50 per cent turnout from a party membership that swelled to over 131,000 during the leadership campaign.
About 56,000 people had voted in advance of the convention.
Under the preferential ballot system, in which voters ranked their choices first to last, those 56,000 votes were locked in for all subsequent ballots and couldn't be influenced by floor-crossing endorsements.
None of the vanquished candidates except Singh — he, as expected, went with Mulcair — chose to publicly endorse another contender.
As it transpired, key endorsements weren't required.
Mulcair's high profile in Quebec helped him maintain his status as the candidate to beat. Once a western-based protest party, the NDP's world has revolved around Quebec since last May's election, when an unexpected Layton-led wave swept the province and vaulted the party into official Opposition status for the first time in its 50-year history.
As the lone Quebec MP in the hunt, Mulcair made a powerful case as the standard-bearer for a party in which 58 of its current 102 seats came from his home province. Mulcair was the lone New Democrat among those 58 Quebec MPs who held his seat before last May's federal election.
He has also caused divisions, however, among social democrats who believe he'll turn the party into a pale imitation of the more centrist Liberals.
Mulcair said he'll rise to the challenge.
"We can't ask our fellow citizens to do their part for our country if their government will not do it's part for them," he told the crowd.