But it's not just his party that misses him.
While hosting journalists at his home for an annual Christmas party, Prime Minister Stephen Harper said he too feels Layton's absence in the House of Commons.
Their last public exchange had come in June, when they found themselves in Parliament for three days straight as the NDP stalled legislation to end a lockout at Canada Post.
As the debate wore on and wore everybody down, Harper crossed the floor to sit beside Layton and the two chatted amiably for a few minutes.
The prime minister later recalled they talked about both the personal and the political.
Now, there's no one on the Opposition bench to negotiate with, he told reporters clustered in the living room at 24 Sussex earlier this week.
Not that with his majority government he's needed to do any negotiating, he hastened to add.
But with the holiday din of the party around him, Harper's voice softened, saying he also missed Layton not just as a politician, but as a man.
The two shared more than a love for politics. Both were avid musicians.
When Layton died of cancer in August, Harper had admitted a missed opportunity.
"I will always regret the jam session that never was," he said at the time.
"That is a reminder, I think, that we must always make time for friends, family and loved ones, while we still can," he said.
But it's the absence of political jam sessions felt by the New Democrats.
When Layton stepped down as leader of the NDP in July to fight a new cancer diagnosis, he chose new MP Nycole Turmel as interim leader.
While she'd only been a member of parliament for three months, the former leader of the Public Service Alliance of Canada had years of experience working with the federal government.
She had already been appointed the party's caucus chair and at the time, Layton said she had tremendous support within the NDP.
But she didn't know Harper.
"One of the things that I think without Jack that we don't have is a leader with a relationship with the prime minister, who knows the prime minister and knows kind of what things you can negotiate," said Anne McGrath, who served as Layton's chief of staff and now holds the same role with Turmel.
Sitting in on meetings between Layton and Harper, it was clear they did understand each other, McGrath said.
"There was a certain level of respect there," she said.
"They both understood politics too and they understood what the purpose of those meetings were and where you could reach across partisan divides and where you just can't."