OTTAWA -- The Ontario Federation of Labour is throwing unequivocal support behind Tom Mulcair's NDP in the federal election campaign.

And OFL president Sid Ryan is urging the entire labour movement to unite behind the NDP in a concerted effort to defeat Stephen Harper's Conservatives.

But while other labour unions share the federation's determination to get rid of the Conservatives, not all think giving a blanket endorsement to the NDP is the best way to go about it.

Unifor, the country's largest private-sector union, is endorsing incumbent New Democrat MPs but urging its members to vote strategically in other ridings for whichever party has the best shot at defeating the Tories.

The OFL's counterpart in Quebec, the Federation des travailleurs et travailleuses du Quebec, has taken the same anybody-but-Conservative stance as Unifor.

The OFL itself adopted a more cautious strategic voting position during last year's Ontario election, in a determined bid to stop the Conservatives' Tim Hudak; but Ryan says that caution is not needed federally because the NDP is the front-runner this time.

"I think this time that's all been turned on its head, given the strong numbers the NDP are coming up with in poll after poll," Ryan said in an interview.

He predicted a united front by unions will help create an unstoppable wave for the New Democrats, such that even initially hopeless ridings will wind up being snagged by the NDP.

"It's why we are endorsing them wholeheartedly and saying if the entire labour movement gets in behind all the candidates for the NDP, we have a real shot at pulling along some of those folks that are today maybe not as clearly favourites in their own particular ridings."

In a news release Friday, Ryan warned: "Now is not the time to divide the strength of the labour movement by sending out confusing messages about who should be the next prime minister of Canada."

Unifor president Jerry Dias said his union also wants to defeat the Conservatives but doesn't buy the OFL's tactic.

"The only difference with us and maybe some of the others is if I have my choice in a riding of Conservative or Liberal, I'll take a Liberal. I won't risk (voting NDP so) that a Conservative gets elected because of vote splitting," Dias said.

While national polls can give voters a sense of which party is ahead overall, they are often of little use to strategic voters in individual ridings where a host of local factors produce quite different standings for the parties. Hence, an NDP voter in a riding where the contest is really between the Conservatives and the Liberals could actually be helping the Conservatives; likewise a Liberal voter in riding that's a Tory-NDP contest.

To help strategic voters make educated decisions, Dias said Unifor is conducting polling on a riding-by-riding basis across the country. It will continue to do so up to election day on Oct. 19, so its members have the most up-to-date idea of which party is best poised to defeat the Tory candidate in each member's riding.

"We are, I would suggest to you, going to step it up several notches from any of our previous (campaigns). We are taking our politics very serious because of, frankly, the attack on the labour movement but the attack on working class people generally by this government," Dias said.

"I mean, it's a disaster."