The NDP's caucus retreat in Montreal this week comes at a time when the party is at its lowest level of public support in over a decade.

One year ago today, in the throes of the federal election campaign, the New Democrats were on pace to win. They were leading in the polls with about 32 per cent.

But since last year's vote, in which the NDP captured 19.7 per cent of ballots cast and was reduced to third-party status, the party has averaged just 13.7 per cent in the polls.

The New Democrats have not consistently polled so low since 2003 in the first months of Jack Layton's leadership, and they are below Layton's worst electoral performance in 2004. The NDP took just under 16 per cent of the vote in that campaign.

At current levels of support, the NDP is instead flirting with the kind of results the party put up between 1993 and 2000, when the Liberals under Jean Chrétien were elected with majority governments. A weak NDP is key to Justin Trudeau repeating Chrétien's success.

Regional bulwarks collapsing

This drop in NDP support has been felt throughout the country, but particularly in the two provinces where the party put up its best numbers last year. The NDP took 26 per cent of the vote in British Columbia and 25 per cent in Quebec. More than two-thirds of its caucus of 44 MPs were elected in these provinces.

Since the election, however, the NDP has averaged just 17 per cent in both B.C. and Quebec. In B.C., the NDP has scored less than its 2015 performance in 30 of 31 polls. In Quebec, it has scored below that bar in every poll published since the last election.

A year ago today the NDP was at 36 per cent in British Columbia. The New Democrats were at 45 per cent in Quebec and led the Liberals by more than 20 points.

NDP election results

Share of the popular vote taken by the NDP between 1993 and 2015, along with its average support in the polls since the 2015 election. (CBC)

The reason now is a tough time to be a New Democrat is that the party is losing its traditional base — the left of the political spectrum — to the Liberals, who are sounding and looking like a progressive party intent on squatting on the NDP's territory.

According to Abacus Data, 72 per cent of Canadians who self-identify as being on the left say they approve of Trudeau's government. The polling firm also found that 21 per cent of Canadians who voted for the New Democrats in 2015 would now cast a ballot for the Liberals.

Forum Research put that number at 44 per cent. It also found that a majority of NDP supporters approve of Trudeau and that 25 per cent said he would make the best prime minister — compared to 39 per cent who said their own party's leader, Tom Mulcair, would be the better man for the job.

Mulcair's ratings dip

Mulcair, who may be facing challenges to his leadership from within the NDP caucus, has seen his approval ratings dip since the election. According to the CBC's Leader Meter, Mulcair has averaged an approval rating of 36 per cent over the last 10 polls, with 38 per cent disapproving. That compares poorly to the 50 to 31 per cent split he managed during the election campaign.

Nevertheless, Nanos Research finds 46 per cent of Canadians think Mulcair has the qualities of a good political leader, compared to 37 per cent who don't. But the same poll found that just eight per cent of Canadians prefer him for prime minister. Trudeau scored 54 per cent.

Leadership vacuum

The Liberals, who hold a wide lead in the polls, might be benefiting from the lack of leadership at the head of both the Conservatives and NDP. The Conservatives won't name their new permanent leader until next spring, while the New Democrats will choose Mulcair's successor in October 2017. Perhaps Trudeau and his party shine by comparison to the vacuum on the other side of the aisle.

But the difficulties the New Democrats are having seem to be something else entirely. The leaderless Conservatives have only seen their support decrease by three points since the election. The New Democrats have seen their support drop by seven points, or more than a third of the vote they had last fall.

The last time the NDP was leaderless after Layton's death in August 2011, the party did not see the same kind of collapse. Then in the Official Opposition role, the New Democrats only saw their support dip by three points.

Temporary love

The Liberals after the 2011 federal election, under interim leader Bob Rae, were always more popular with a temporary leader than they were when Michael Ignatieff was last at the helm.

Polls suggest the New Democrats still have enough accessible voters in their reach to win an election. But they also show that those accessible voters are, for the most part, sticking with the party in power for the time being. 

If the New Democrats can't find a way to lure them back, they will be in danger of returning to the days of the pre-Layton NDP of the 1990s and early 2000s. That is a long way to fall from being a government-in-waiting.