If the comeback continues, the leaders' debates will stand out as the moments he saved his party's campaign.
But if his opponents took notice of him some days earlier, at the sugar shack in Quebec, they might have known that Jack Layton was not going to let himself be pushed to the sidelines of this federal election much longer - that, having kick-started the campaign by opposing the federal budget, he would yet play a very big role in shaping the next Parliament.
The early weeks of the campaign were fairly brutal for the NDP Leader. Nationally, it was increasingly being cast as a two-way race between Conservative Leader Stephen Harper and Liberal Leader Michael Ignatieff. Struggling to get media attention, Mr. Layton saw his poll numbers slide. Recovering from hip surgery, facing constant questions about his health following recent cancer treatment, he seemed to be labouring from event to event - a shadow of his usual energetic self.
By the campaign's second weekend, Mr. Layton could have just been going through the motions, trying to minimize his party's seat losses before handing over the leadership after the election. He could have sulked a little, and nobody would really have blamed him.
Instead, to the sound of Québécois folk songs, Mr. Layton danced a little jig.
It was a small moment. But it epitomized what has made him such an enduring presence on the national scene - and what allowed the NDP to emerge from the campaign's third week not just back in the fight, but as the party with the most discernible sense of momentum.
Of the federal leaders, Mr. Layton is the one who most clearly loves his job. At his best he conveys that his enthusiasm is driven by a genuine passion for his social democratic ideals. But there can be no mistaking that he likes the theatre of politics, enjoys being centre stage, and is comfortable enough in his own skin to show some humanity.
Even in the campaign's early weeks, there were signs of that personality shining through. Rather than bristling at his unfriendly media coverage, he took his guitar to the back of his plane for what has become a traditional sing-along with journalists. At another point, he committed a bit of a flub, referring to "the ghost" of the very much still alive Jean Chrétien. But rather than get his back up, or retreat into spin, he immediately laughed at his own mistake and apologized.
Had many voters seen those moments, they might have helped Mr. Layton's cause. And in Week 3, when they finally did get exposed to him in large numbers, he made the most of the opportunity.
To some eyes, particularly during the English-language edition, Mr. Layton's debate performance grated. His reference to "crooks" in the Senate drew an audible gasp from the live audience - the only sound heard from offstage that night. And dropping "bling" and "hashtag fail" into a single response seemed a shameless and misguided attempt to connect with youth.
But he also managed to combine aggression with good humour in a way that worked with audiences. He was more sharp-tongued than the other leaders, but also the only one to joke and smile. Meanwhile, he looked confident and well rested - a far cry from the sickly specimen viewers had been conditioned by some coverage to expect.
Mr. Layton's main objective was to make himself impossible to ignore - not just those two nights, but going forward. And not only did he get noticed; he got noticed favourably. Polls showed that viewers considered his performance much stronger than that of Mr. Ignatieff, his main competitor for votes.
More importantly, Mr. Layton has at least temporarily reversed his party's slide in popular support. By last weekend, the NDP had slid below 15 per cent nationally. Now, it's trending upward toward 20 per cent. Meanwhile, Mr. Layton's "leadership index score" (a Nanos Research measurement that takes into account perceived competence, trustworthiness and vision) has spiked sharply - still nowhere near Mr. Harper's level, but well above Mr. Ignatieff's.
If those numbers hold, it stands to have a huge impact on the results on May 2, obliterating the assumptions and the strategy of one other party in particular.
For the Liberals, this campaign was supposed to be all about winning back voters who have abandoned them for the NDP over the past decade. Toward that end, their platform leans heavily leftward; with the exception of Afghanistan, the differences between the two party's positions are not easy to spot.
In theory, the strategy makes sense: It's to the NDP, rather than the Conservatives, that the Liberals have bled the most votes. But Mr. Layton's personal appeal, relative to Mr. Ignatieff's, shows signs of rendering the Liberals' leftward swing useless.
For the NDP, meanwhile, what was once an election about minimizing losses might now be about making new inroads.
In Ontario, where the most ridings are up for grabs, the New Democrats' biggest effect may be to draw votes away from the Liberals, helping elect more Conservatives. But in British Columbia, helped by being the only federal party to oppose the unpopular harmonized sales tax, there's some room for gains. And in Quebec, the NDP's flirtation with soft nationalism - combined with social values that play well in Montreal - give it some chance of besting the Liberals and Conservatives as the alternative to the Bloc Québécois.
Among New Democrats, Mr. Layton's performance lately - combined with Mr. Ignatieff's struggles - has revived the faint hope of moving toward replacing the Liberals as the Conservatives' primary rivals. That has been Mr. Layton's dream since taking the party's helm, and it would be a legacy for the ages.
But for all the optimism, there is a catch. This may well be Mr. Layton's last campaign as leader; in any event, he won't be around forever. And it's him, more than his party's brand, that many Canadians seem to have warmed to.
Without another leader with his resilience, and his charm, the NDP might wind up back where it started. It's not every leader who dances a jig on a newly repaired hip. But for now, New Democrats are just grateful they have one who does.
With reports from Patrick Brethour and Les Perreaux