I have been hearing of torrent of criticism and putdowns against Justin Trudeau and the Liberals from NDP friends and acquaintances for months. Attacks such as “airhead” and “moron” and “unprincipled” have been echoing on my Facebook timeline and Twitter, mimicking attacks from the Conservatives in their well-financed ad campaign.

But I say those lefties should tone it down a bit and re-focus their fire on the real enemy: Stephen Harper. Why? Because the NDP needs the Liberals to make significant gains in this election. Yes, you read that correctly.

I reject the notion that two strong centre-left parties will just divide the anti-Tory vote. That’s partially true. But a stronger Liberal Party with much more than the paltry 19% they took in 2011 will rob the Conservatives of the votes they need to slip into another majority.

Blue Liberals, red Tories and centre-right folks tired of Stephen Harper’s ways will probably still end up voting for him if the only alternative seems to be the NDP. That’s what happened in 2011. I’ll be damned if the same thing happens again this year.

To avoid this, those centre-right folks (especially in the huge Greater Toronto Area) need a strong Liberal option that appears competitive. There are literally dozens of ridings across the country where the Liberals will be the main competition against the Tories. The Grits remain the strongest challengers there with great local candidates. I have a sick feeling that should this federal election polarize too strongly between Harper’s Conservatives and Mulcair’s NDP, it’ll only benefit the former in those dozens of ridings and push the Tories toward another majority. Clearly with their continued attacks on Justin Trudeau, that’s what the Conservatives are hoping.

Sure, the NDP dreams of winning over masses of voters in the 905 area of Toronto and beyond, taking seats they’ve never come close to taking in the past. The NDP did win 15 out of 25 seats in Calgary in the May provincial election, after all. Certainly they can do the same elsewhere, some insist.

No, they won’t. First, the Alberta NDP was the only serious centre-left option in that fight. The Alberta Liberals and the little Alberta Party combined won only 6% of the vote. The Alberta NDP also benefited from a divided right. We have the opposite situation federally, with a strong Liberal Party sharing the opposition vote. Plus Harper’s Conservatives aren’t anywhere near as detested as the Alberta PCs. They are still tied for first place in two credible polls out this week and are very much in the federal game. Their best hope remains suppressing opposition support while re-inspiring their base to turn out.

Yes, Trudeau’s Liberals have suffered a correction since last year in the polls. The shine is definitely off Justin’s leadership rose. His inexperience and lack of gravitas have left many progressives hoping for change turning to the more experienced, tougher Tom Mulcair.

I do believe that Mulcair’s NDP will form the greatest overall threat to the Conservatives this year, but not in every single riding. The NDP’s new support in the low 30s is not about to decline. Trudeau’s mistakes on Bill C-51 were the straw that broke the camel’s back. Those votes are lost to the NDP this October. The NDP may even be able to push their vote up from 31% last time but I’m not predicting it’ll get much higher than 33% or 34%, unless of course Harper’s campaign truly goes off the rails. If a higher Grit vote can pull the Tories down from 40% last time to 32% or 31% or even lower this time, that’ll help the NDP win the most seats.

I do think Trudeau still has considerable abilities and appeal. I don’t usually like to link to Warren Kinsella, but he’s bang on here. He’s also largely correct here explaining the Liberals’ fall from grace and how Justin may fix the situation.

With a stronger Liberal caucus this year, perhaps two or three times its current size, the result will either be a minority Conservative government or a minority NDP government. And that’ll spell a quick end to Stephen Harper’s reign as there’s no way the Trudeau Liberals will allow Harper to survive longer than his new Throne Speech. While I doubt we’ll see a formal NDP-Liberal coalition as some would like, I’m sure some working arrangement similar to the 1985 Peterson-Rae accord will be in the works.

That would be a government to celebrate. So lefties, please lighten up on the anti-Trudeau attacks. You’re doing Harper’s bidding.

Follow Matt Guerin on twitter: @mattfguerin