Justin Trudeau's approval ratings have dropped to their lowest level since he became prime minister, according to a new poll. But an analysis of the popularity of his predecessors suggests Trudeau's sliding numbers are typical of a prime minister roughly 15 months after taking office.

The survey, published by Forum Research for the Toronto Star, found Trudeau's approval rating sitting at 48 per cent, down three points since December and 10 points since November, with his disapproval rating increasing to 42 from 32 per cent two months ago.

Those are the worst numbers Forum — or any other pollster — has registered for Trudeau since he became prime minister.

Support for the Liberal Party has also slipped, with Forum pegging it at 42 per cent. That is unchanged from December but down nine points from November. A four-week rolling poll by Nanos Research found Liberal support down to 39.4 per cent, a decrease from 43.6 per cent in the previous four-week sample.

This drop in support coincides with questions raised about the Liberals' fundraising practices and the prime minister's vacation in the Bahamas, where he stayed on the private island of the Aga Khan. Canada's ethics commissioner has launched an investigation of the trip.

Trudeau ranks in the middle of past PMs

But while Trudeau's drop in support is a shift from the consistently high numbers he and his party enjoyed during his first year in office, it is not unusual from a historical perspective.

Of the eight last prime ministers who were in the job long enough, and for whom polling data exists, Trudeau's approval rating ranks as the fifth highest compared to where his predecessors stood 15 months after taking office.

Approval ratings of prime ministers about 15 months after first taking office. Blank space in charts denotes undecideds or those without an opinion. (Scott Galley / CBC News)

In the spring of 2007, Stephen Harper's approval rating was 49 per cent, according to Environics, which conducted polls regularly over the period stretching from Brian Mulroney's mandate through Harper's. And 15 months after replacing Jean Chrétien, Paul Martin had an approval rating of 48 per cent — matching Trudeau's.

The current prime minister's disapproval rating is five points lower than either of his two predecessors at this stage of their mandate, however.

Trudeau's approval rating is one point less than Lester Pearson's was in a Gallup poll from 1964, but Pearson's disapproval score was just 28 per cent, significantly lower than the current prime minister.

Higher approval than his father

Trudeau's rating is higher than Mulroney's was 15 months after he came into office (43 per cent). He also beats his father's approval rating in 1969, when Pierre Trudeau had just 34 per cent approval (but a disapproval rating of 38 per cent, also lower than his son's).

Jean Chrétien and John Diefenbaker, however, were doing much better than Justin Trudeau a little over a year after first taking office. In early 1995, Chrétien had an approval rating of 63 per cent — equal to Diefenbaker's approval rating in 1958.

Diefenbaker was in the midst of an electoral honeymoon, though. He had initially come to power after winning a minority government in 1957 but won a massive majority in early 1958, well before he had reached the 15-month mark in office.

An uncertain future

Where past prime ministers' approval ratings went after the 15-month mark varied significantly. Both Harper and Martin saw their approval ratings continue a slow decline. Chrétien's strong ratings held until 1996, about three years after coming to power, then rebounded after the 1997 election and stabilized until he left office in 2003.

Mulroney saw his approval ratings continue to collapse after 1985, rebound during the free trade election of 1988 and then collapse again to historic lows — just 12 per cent of Canadians approved of the former Progressive Conservative leader in 1992.

Prime Minister Brian Mulroney's approval rating dropped to 12 per cent in 1992. (Chuck Mitchell/Canadian Press)

So by historical standards, things could be better or worse for the Liberals. But those standards also suggest that the prime minister's approval rating 15 months after coming to office provides little indication of where that rating will be in the future. Mulroney and Pierre Trudeau, the worst performers, both secured re-election — multiple times in the case of the elder Trudeau.

Of the best performers, Diefenbaker lost his majority in the next election and Pearson never secured anything other than a minority government.

The record suggests that an approval rating on a negative trajectory is not necessarily a harbinger of trouble to come for a sitting prime minister — it is instead rather typical. From their post-election highs, Harper, Martin, Chrétien and Mulroney saw their approval ratings drop by an average of about 10 points. That is roughly equal to the decrease in support Trudeau has seen.

If Trudeau continues to follow the trend, that means his approval rating is likely to continue sliding — just as his predecessors' ratings did over the course of their second years in office.

The poll by Forum Research was conducted between Jan. 19 and 21, 2017, interviewing 1,332 Canadians via interactive voice response. A probabilistic sample of this size would yield a margin of error of +/- 2.7 percentage points, 19 times out of 20.

The poll by Nanos Research was conducted between Dec, 18, 2016 and Jan. 20, 2017, interviewing 1,000 Canadians via the telephone. A probabilistic sample of this size would yield a margin of error of +/- 3.1 percentage points, 19 times out of 20.