OTTAWA -- Two unions top the list for third-party ad spending this campaign season, as groups look to make their mark while sticking within the limits set by Canada's new spending rules.

Documents filed with Elections Canada show Unifor has shelled out over $1.2 million while the United Steelworkers spent $736,349 since the start of the pre-election period on June 30 and through the first week of the official campaign period.

Unifor's president Jerry Dias said Wednesday the union would "spend every dime we're entitled to" in order to get its anti-Conservative message out to its 315,000 members.

The country's largest private sector union is running a national-level campaign of TV, radio and social media advertising.

Dias was forthright about his desire to do even more to avoid a Conservative government, saying he wished there weren't limits on third-party spending.

"If I could have spent another $1.2 million during the writ period, I certainly would have," he said.

Third-party groups are allowed to spend just over $1 million in the pre-writ period, and slightly over $500,000 in the campaign itself.

Dias said the over $900,000 it spent in the pre-writ period is already having an effect, claiming partial credit for what polls suggest is a decline in support for the federal Conservatives from a high in early June to a dead-heat now with the governing Liberal.

The Steelworkers, meanwhile, with 225,000 members in numerous sectors besides steel, produced a mix of TV and radio campaigns in support of the NDP.

Ken Neumann, the union's national director, said he supports the limits on third-party advertisers, noting the advantages of the Canadian system over the few restrictions on third-party spending in the United States.

Still, Neumann said it's important to be part of the political conversation.

"I truly believe we should be part of the democratic discussion on behalf of our members," he said.

The two unions are far and away the biggest players to disclose expenses, together making up more than three-quarters of spending published so far on Elections Canada's website, with more returns still to be published.

But there are other third-party groups on the board as well, including a mix of business groups, unions and environmentalist organizations.

Canada Growth Council, a pro-energy, pro-free enterprise group has spent almost $100,000 so far, targeting Liberal politicians with ads across Western Canada.

A spokesperson for the group, Derek Robinson, said it has so far put up 60 billboards across the country. He said the group will likely shift to digital and mailed ads focused around the Greater Toronto Area.

One reason for that is the per-riding spending limits in place during the election. Once the campaign started, third-party groups were limited to about $4,400 for campaigning activities targeted in any given riding.

"That's a tiny amount -- that could be two billboards depending on where you put them," Robinson said.

He said he found the per-riding restrictions during the writ period to be "extremely limiting."

"I'm not one of the people that finds this is limiting our freedom of speech," Robinson said, adding the rules do make it hard to have a strong voice.

He also said it was time-consuming to comply with the new third-party disclosure rules.

Near the other end of the political spectrum, North99, which started as a Facebook page just a few years ago, is pushing an anti-Conservative message during the campaign.

The group has spent just over $20,000 so far, with most of the advertising purchases on Facebook. Future buys will be deployed "where we think we can make the biggest difference," said Taylor Scollon, a co-founder of the group.

Scollon also praised the new disclosure rules in Canada meant to provide transparency around third-party spending.

"I think it's a shame they only exist during the campaign period," he said.

Canada actually has fairly strong regulations on third-party advertising compared to other countries, according to Michael Pal, a political science professor at the University of Ottawa.

Pal said "more transparency is generally preferable," noting in particular more real-time disclosure would be helpful, and that rules to prevent collusion between political parties and third-parties should likely be updated.

He said it should be interesting to see how much groups are spending in the pre- and post-writ periods, and how new groups that originated in online platforms like Facebook fit into a third-party landscape that has traditionally featured an array of environmental, labour and corporate groups.

One group absent so far from the disclosures is Canada Proud, the federal offshoot of Ontario Proud, a third-party group that spent almost $450,000 on the 2018 provincial election in Ontario.

Third-party groups must start filing interim returns with Elections Canada once they spend or receive over $10,000 for regulated political activity.