Justin Trudeau has given the Liberals new optimism for the future, as the support for their party surges now that he has become leader. But based on how other new leaders have performed over the past 30 years, he will need to be one of the exceptional success stories if he is to win the 2015 election.

In the first quarter of 2013, the Liberals were averaging about 25 per cent in the polls. That has increased to 31 per cent in the first weeks of April, including surveys taken just before and after Mr. Trudeau was officially named the new leader of the Liberal Party.

A gain of six points from the quarter preceding a leadership victory is not unusual. On average, new leaders going back to Brian Mulroney have been about three times more likely to improve their party's polling numbers than to hurt them. On average, parties have gained 1.8 points from one quarter to the next when a new leader is named – and 3.1 points if the exceptional case of Jean Charest's first months at the helm of the decimated Progressive Conservative Party in 1993 is excluded.

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Subsequently, however, the initial boost in support has tended to wear off. New leaders have lost an average of 3.3 points in the quarter following their leadership wins. By the time the next election rolls around, that has increased to 4.6 points. New leaders have been three times more likely to lose support by election time than gain it, compared to where they started in the polls as party leader.

For Mr. Trudeau and the Liberals, that suggests that his party's support could drop to around 28 per cent by the summer and 26 per cent by the 2015 election. Though that is far better than the 19 per cent the party took in the 2011 election under Michael Ignatieff, it would be a disappointing result compared to the party's current level of optimism.

But Mr. Trudeau may not be an average leader. By already boosting the party's support by six points, that puts him above the performance of Jack Layton and Stephen Harper when they initially became new leaders in 2003 and 2004, respectively. But it also puts him below such Liberal flame-outs as Stéphane Dion and John Turner.

However, if Mr. Trudeau is able to keep the Liberals above 31 per cent by the summer he will be in rare company. Over the last three decades, only Thomas Mulcair, Kim Campbell, and Mr. Ignatieff have been able to increase their support in the quarter following their leadership victory. Mr. Mulcair's fate is still unknown, but Ms. Campbell and Mr. Ignatieff may not be examples the new Liberal leader would like to follow.

He could do worse, however. Paul Martin lost 13 points in the quarter following his leadership win, while Jean Chrétien's support fell by 17 points and Mr. Turner's by 25 points. It seems unlikely that Mr. Trudeau will suffer in the same way, primarily since these three former Liberal leaders kicked off their leadership at much higher levels of support (51, 44, and 45 per cent, respectively).

If Mr. Trudeau is to have a hope to win the 2015 election, he will need to be the first Liberal leader in at least the last 30 years to gain support at the ballot box. Mr. Turner's support dropped by 17 points between his leadership victory and the 1984 election, while Mr. Martin's dropped by 14 points in 2004. The performances of Mr. Dion (a drop of six points) and Mr. Ignatieff (a drop of nine points) would also need to be avoided, while Mr. Chrétien's comparatively good decrease of only three points would still keep the Liberals from power in 2015.

He does have a few good examples to follow, however: Mr. Layton and Alexa McDonough both took a little more of the vote in their first election than they had in the polls when they first became leader, while Mr. Charest returned his party from the ashes by gaining 11 points between his standing in the polls in 1993 and the election result of 1997.

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If Mr. Trudeau is in the midst of a honeymoon that will eventually wear off, how long does he have? On average, it has taken about nine months for a party's support under a new leader to fall back to where it stood before he or she was given the reins. That puts the Liberals back down in the high-20s by early 2014.

But not all honeymoons come to an end. Before Mr. Harper won the leadership of the Conservative Party in 2004, the leaderless Tories at the end of 2003 were polling at around 26 per cent. The party has never consistently polled at such a low level of support since he took over the leadership. The New Democrats, too, consistently polled at higher levels of support than the 12 per cent the NDP had just prior to Mr. Layton's leadership victory in 2003.

By one measure, Mr. Trudeau does stand out from other leaders over the last three decades. He has already boosted Liberal support by some 12 points since the last election. That is a greater increase in support under a new leader from the previous election than any other leader over the last thirty years, with two exceptions. When Mr. Mulroney took over the Tories in 1983, support for the party was 21 points higher than it had been in the 1980 election. And when Mr. Chrétien became Liberal leader in 1990, Liberal support was 12 points higher than where it was in the 1988 election. These are two examples worth following. Just maybe, the newfound optimism in the Liberal ranks is not completely misplaced.

Éric Grenier writes about politics and polls at ThreeHundredEight.com .