Cannabis has become a battleground between Canada's Liberal and New Democratic parties as the 2015 federal election enters the home stretch.

On Oct. 14, the Liberals attacked Tom Mulcair's shifting stance on legalization by digging up an old quote in which the NDP leader opposed decriminalization: "I think that would be a mistake," Mulcair told Tom Clark of Global TV in March 2012, "because the information that we have right now is that the marijuana that's on the market is extremely potent and can actually cause mental illness."

The next month - on April 20, coincidentally - NDP spokesperson George Soule said, "Thomas Mulcair does not believe that anyone should be going to jail for possession of just a small amount of pot. Criminalization is not the answer for any area of social policy."

Why was there confusion? "Terms like legalization and decriminalization are often inappropriately used interchangeably," Soule explained.

The NDP has been a longtime proponent of decriminalization, so Mulcair's comment put himself offside with his own party. This is something Liberal leader Justin Trudeau should understand as his position on cannabis also clashed with his party's, once upon a time.

Indeed, Trudeau's anti-decriminalization stance in 2010 was strikingly similar to Mulcair's in 2012: "It's not your mother's pot," he told Macleans, noting that cannabis in the 21st century is significantly stronger than the buds smoked decades ago. "I lived in Whistler for years and have seen the effects. We need all our brain cells to deal with our problems."

Sounds like Trudeau in 2010 and Mulcair in 2012 had about as much common ground on cannabis as they do in 2015.