NDP Leader Tom Mulcair received a bit of good news this week, getting the public support of the leaders of five big unions on Tuesday.

At the party's convention in Edmonton on April 8-10, however, Mulcair will require more than that: he'll likely need the support of a significant majority of the party's rank and file to stay on as leader.

Knowing how the NDP's membership, let alone the 1,500 delegates who will be making the trip to the Alberta capital, would vote on the question of Mulcair's leadership is virtually impossible. But polling conducted since he became leader in March 2012 does provide a glimpse of how NDP supporters feel about Mulcair.

Polling regularly conducted by Forum Research suggests that Mulcair's support among NDP voters increased through to the 2015 election, before dropping back to virtually Square 1 following his party's defeat in October.

The evolution of Mulcair's approval ratings among NDP voters can be divided into five stages: the early days of his leadership; the period surrounding Mulcair's grilling of then prime minister Stephen Harper over the Mike Duffy scandal; the last months before the campaign began; the campaign itself; and the time since then.

Between March 2012 and August 2013, Mulcair's approval ratings among NDP supporters was relatively low for a sitting leader — largely because of the significant proportion of NDP voters who had yet to form an opinion of him.

In this period, Mulcair averaged an approval rating of 61 per cent and a disapproval rating of 14 per cent, with one-quarter of NDP voters unsure of their opinion.

By contrast, over the same period (and after he became Liberal leader) Justin Trudeau averaged an approval rating of 84 per cent among Liberal voters. Only 12 per cent, on average, were unsure, and just five per cent disapproved of him.

Pre-election peak

Mulcair's approval ratings saw an uptick in the wake of the Duffy scandal, as the then opposition leader led the news on a virtually nightly basis. His numbers among NDP voters also improved, and he averaged 67 per cent approval between September 2013 and January 2015 as his "unsures" dropped to 21 per cent and his disapproval rating to 12 per cent.

However, it was not until 2015 that his approval ratings among New Democrats moved above the 70 per cent mark, a threshold that has been mentioned by some as the minimum Mulcair will need to meet at the party's convention in order to have the authority to stay on (technically, he needs just 50 per cent plus one vote).

From the beginning of 2015 to the start of the election campaign in August, Mulcair's approval rating jumped to an average of 76 per cent among New Democrats, with his disapproval rating falling to single digits.

New Democrats' esteem for their leader increased as the party moved into first place in the polls. Mulcair averaged an approval rating among NDP voters of 83 per cent during the election campaign, with disapproval at just six per cent and "unsures" at 12 per cent. Those were the best numbers Mulcair has put up to date.

They came tumbling down after the NDP was returned to third-party status on election night. Since then, Mulcair's approval rating has averaged 69 per cent, putting him back down below the 70 per cent threshold. Alhough that is still better than where he was in the early days of his leadership, his average disapproval rating of 15 per cent is narrowly the highest it has been since he first became leader.

Layton's numbers little better

That Mulcair is hovering around the 70 per cent threshold among his own party's supporters in national polls means next to nothing for the leadership vote in Edmonton. In fact, it didn't hurt Jack Layton in the past at all.

In polling conducted by EKOS Research in 2009 and 2010, when the New Democrats had roughly about the same level of support as they do today, Layton was scoring an approval rating of between 65 and 70 per cent among NDP voters. His disapproval rating sat at between 11 and 13 per cent — little different from Mulcair's current averages.

That did not prevent Layton from registering 89 per cent support in a leadership review in 2009.

Of course, the context was different. The New Democrats were not coming off a stinging defeat after flirting with power. And they were up against an unpopular Liberal leader in Michael Ignatieff.

Trudeau, by contrast, has higher approval ratings than Mulcair currently does among all Canadians. Among Liberal voters, Trudeau's approval rating has averaged 90 per cent since the election, with just three per cent of Liberal supporters disapproving of him.

Mulcair would likely feel much more comfortable going into his party's convention if he had the same kind of numbers among his own party's supporters — even if they aren't necessarily the ones who will make the journey to Edmonton next week.

This article uses averages derived from a number of polls conducted by different poll firms, which differ in terms of methodology, sample size and field dates. These surveys have not been individually verified by the CBC.