Politicians know how to harvest the low hanging fruit at election time.

Now, Justin Trudeau has sniffed out the low hanging cannabis on the campaign trail — and promised to legalize it.

It’s time, long past time. The Liberal leader can make up for the sins — or omissions — of his father in failing to decriminalize marijuana possession generations ago, when Pierre Trudeau ignored the recommendations of the 1973 Le Dain Royal Commission he created as prime minister.

In fairness to Trudeau the elder, it was a different time. In the decades since, Canada has decriminalized homosexuality and legalized gay marriage.

In this campaign, the NDP is proposing to decriminalize dope. The Liberals want to take it one step further by legalizing — regulating — marijuana for sale in pharmacies (much like booze, still banned for minors).

Conservative Leader Stephen Harper has pounced, claiming Trudeau will imperil young people with poison (better to leave it to the dealers?). NDP Leader Tom Mulcair mocks his Liberal rival as a callow opportunist (better to maintain prohibition?).

We’ve just seen a variation on this theme play out in Ontario. A province still suffering from its post-Prohibition hangover — and hang-up — beer sales were long restricted to a foreign-controlled monopoly.

Liberalizing beer sales remained a social taboo for politicians, creating perennial paralysis. None of the major parties would touch the topic during last year’s election.

In retrospect, it was a missed opportunity for then-PC leader Tim Hudak. Ahead of the campaign, he had targeted the Beer Store, promising to rescind the chain’s outdated monopoly.

But at election time, he lost his nerve — and lost his way. Hudak himself now says he feared being belittled by his opponents for stooping to election gimmicks like beer in corner stores (just as the Conservatives now attack Trudeau, falsely claiming we’ll have cannabis in corner stores).

Now he’s watching Premier Kathleen Wynne, who barely spoke of beer during the campaign, stealing his ideas by phasing in sales to 450 grocery stores. Imitation is the sincerest form of flattery, but for Hudak it’s a bitter bottle of beer to swallow.

Beer surely wouldn’t have been a vote-determining issue, but it might have helped Hudak break through the static of a campaign when a candidate is constantly under attack. Equally for Trudeau, it’s hard to know how his marijuana position will influence voters, but it’s helping him get noticed with a younger demographic — and forcing the rest us of to take stock.

Prohibition ended in Ontario in 1927, when lawmakers realized a complete ban on booze wasn’t working. It has taken until this year to start liberalizing the rules for selling beer.

Prohibition of marijuana came in 1922, when it was banned without any Parliamentary debate or scientific research. And it has taken until now to get serious about legalizing it, even though we’ve long known that prohibiting dope is a dopey idea.

As a 2002 Senate report noted, marijuana Prohibition was “a solution without a problem.”

Kids get it from local dealers, then grow up to be politicians who, in power, ask the police to throw kids in jail for doing what they once did. A clear sign that politics confuses hallucinatory with hypocrisy.

In 2013, Canada’s police chiefs declared that enough is enough: As a practical matter, they recommended that police merely hand out tickets for possession, rather than locking people up. It was a clear rebuke by police of the “Safe Streets” Act passed by Harper’s government the year before, which set mandatory minimum jail terms (26,000 Canadians were charged with simple possession in 2012 — compared to two women in niqabs at citizenship ceremonies, sparking the latest prohibition craze).

Out of phase with modern policing, the Conservative Party is also out of sync with modern science. Harper has responded to Trudeau’s platform by claiming that “marijuana is infinitely worse” than tobacco, based on “growing scientific and medical evidence” — when the facts from all sources clearly shows the opposite.

As the Canadian Medical Association Journal reminds politicians in its latest election issue, banning dope merely fuels an illegal drug trade that does more harm than good. It wastes more than $1.2 billion a year in law enforcement resources, stigmatizes 500,000 people with criminal records because of cannabis possession, and drives drug users underground.

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Just as Prohibition flopped for booze, criminalizing cannabis is a war without a cause. We’ve known this since the seventies, but allowed anti-drug hysteria to cloud our thinking.

Time, at last, for Canada’s politicians to stop hallucinating and hyperventilating about marijuana. Time, in fact, to take a deep breath — even if they claim never to have inhaled — and come to their senses about dope.