As Liberals gather in Edmonton this week for their summer caucus, a new poll shows Justin Trudeau has widened his lead over Stephen Harper on who Canadians prefer to be prime minister.

The poll, one of the questions of the weekly Nanos Party Power Index that surveys 1,000 Canadians every month and was last out of the field on August 15, found that 31.8 per cent preferred Trudeau for the top job. That compared to 25.5 per cent for Harper, 18.7 per cent for Thomas Mulcair, and 6.3 per cent for Elizabeth May.

Trudeau and Harper have generally been in a much closer contest in Nanos' rolling four-week survey. Over the last 12 months, Trudeau has averaged 29.6 per cent support against 28.6 per cent for Harper. But Trudeau is now more than six points ahead of the prime minister.

And it is not just that Harper is performing below average — he is also near his lowest level of support over the last year (25.2 per cent). The Conservative leader has been plunging of late, as three months ago it was Harper who had 31 per cent support on who Canadians preferred as their leader.

Mulcair is currently polling slightly above his average of 17.9 per cent, but has been unable to close the gap significantly over the two other leaders.

But on another score, the New Democrats are polling above the Tories. Just 36.1 per cent of respondents said they would consider voting for the Conservative Party, a ceiling that puts them well below their electoral performances of 2008 and 2011. By contrast, 44.3 per cent said they would consider voting for the NDP, much better than the party’s all-time best result of the last election.

The Liberals, however, are well above the others with 53.5 per cent of Canadians saying they would consider voting for the party. That gives the Liberals tremendous room for growth, and undoubtedly that potential is coming from both sides of the political spectrum.

It makes the goal of 170 seats that Liberals touted at their summer caucus — the number required to form a majority government — a plausible one. Of course, Liberals are not going to get the vote of every Canadian who says they would consider voting for the party. But 40 per cent, for instance, would make a majority government possible and is well below the current ceiling recorded in the Nanos poll.

The New Democrats, with their ceiling above that 40 per cent mark, can be optimistic. Enough Canadians would consider voting for the party to propel it into government, and that is half the battle. But Mulcair is still trailing Trudeau at length on who would make the best prime minister. While Mulcair is behind on this score, voters who would consider voting for either the Liberals or the NDP will default to Trudeau's party.

Éric Grenier taps The Pulse of federal and regional politics for Huffington Post Canada readers every week. Grenier is the author of ThreeHundredEight.com, covering Canadian politics, polls and electoral projections.