Construction industry unions, the lobby group for big landlords, and the association of insurance companies are the top donors to Ontario's political parties, according to research by CBC News.

With Premier Kathleen Wynne vowing to ban corporate and union donations starting next January, CBC News compiled data from Elections Ontario showing donations to the Liberal, Progressive Conservative and New Democratic parties since 2013.

This is the first time any media outlet has crunched the numbers to reveal a full list of who's pumping the most money into Ontario's political system.

1. United Assoc. of Plumbers & Pipefitters: $491,723

Topping the list is the United Association, the union that represents plumbers, pipefitters and related workers in the construction industry. Individual locals of this union, as well as its provincial council and political action funds donated nearly half a million dollars since 2013, split roughly 70/30 between the Liberals and the NDP.

2. United Brotherhood of Carpenters & Joiners: $484,265

Another union representing construction industry workers comes in second on the list. The Carpenters Union and its provincial district council were the biggest single donor to the Ontario Liberal Party, giving $413,690 to the governing party, nearly eight times the amount given to the PCs.

3. Federation of Rental-Housing Providers of Ontario: $327,665

The association represents the owners and managers of residential rental accommodation. It's been the biggest single donor to the PCs over the past three years, giving $153,392, and nearly as much to the Liberals ($145,550). The bulk of the landlord group's donations went to PC and Liberal leadership candidates.

4. Insurance Bureau of Canada: $315,310

The lobby group for insurance companies split its donations primarily between the Liberals ($152,325) and the PCs ($143,035).

5. GreenField Specialty Alcohols Inc.: $266,640

The ethanol producer and its subsidiaries gave more than 90 per cent of its donations to the Liberals ($249,350), making it the biggest corporate donor to the party and the Liberals' third-largest donor overall.

Rounding out the top 10

Ranked number 6 overall is the Ontario Secondary School Teachers' Federation (OSSTF). It's the biggest donor among the public-sector unions and the biggest single donor to the NDP ($241,065). In seventh spot, nuclear power generator Bruce Power, spreading its $256,991 in donations among all three parties. And rounding out the top 10 are three more unions, Labourers International Union of North America (LIUNA), Service Employees International Union (SEIU) and the Ontario Professional Fire Fighters Association.

The biggest political donor in Ontario is the United Association of Plumbers and Pipefitters, giving $491,723 to the three main parties since January 2013. (CBC)

Most unions in the top rank donated to both the Liberals and the New Democrats, but a few gave exclusively to the NDP: including the United Steelworkers, the Amalgamated Transit Union, and CUPE. Several other unions pointedly gave no money to the PCs, including OSSTF, SEIU and the Fire Fighters Association and its affilliates.

The NDP got nothing from several corporations, including GreenField Specialty Alcohols, TD Bank, Aecon Group, Morguard Corp. and EllisDon.

Reaction to CBC's analysis

At Queen's Park on Tuesday, Deputy Premier Deb Matthews touted what she described as her government's efforts to be more transparent about political donations.

"Giving people access to those donor lists in real time is an important part of our democracy, but we also think that going forward we need to ban corporate donations and union donations," said Matthews.

Asked about corporate and union donations to his party, Progressive Conservative leader Patrick Brown focused on the different amounts of money donated to different parties.

"I think what your article does show very effectively is that the amounts are disproportionate," said Brown. "I appreciate you highlighting that, that there's such a significant difference in donations to the Liberal party by those that are doing business with the government. And the question is, why that difference?"

Ontario NDP leader Andrea Horwath also reacted to the list.

"I think that Ontarians would look at that information and be quite disturbed, frankly, by the list, and that's why I think we're having this discussion," said Horwath. "There's no doubt that we have to get the big money out of Ontario politics, the issue becomes, how do we do that?"

How we analyzed the data

Finding the biggest political donors in Ontario is not a simple task. Elections Ontario catalogues every donation separately and does not tally multiple contributions into annual totals for a donor. So the only way to compile total contributions by donors is to plug each individual donation into a spreadsheet, and repeat the process for every major donor.

The analysis by CBC News combines donations made by a given corporation and its subsidiaries into an overall total for the corporation. Similarly, donations from a union's locals and its umbrella groupings of affiliated locals were combined into one overall total for the union.

For instance, the total for the Labourers International Union of North America (LIUNA) includes donations from its locals, its provincial council and its Canadian umbrella agency, the LIUNA Tri-Fund. On the corporate side, the $266,640 total for GreenField Specialty Alcohols Inc. includes donations made in the name of GreenField Ethanol Inc., GreenField Hensall Inc. and GreenField Johnstown Inc.

The analysis includes all donations to the central parties since January 2013, whether for the annual period, byelections or the 2014 election, as well as donations to the candidates in the Liberal leadership race of 2013 and the PC leadership race in 2015. The data do not include donations given to party constituency associations or individual election candidates.

The leadership races account for significant proportions of the top donors' totals, as there are no limits to how much any corporation or union can make to a leadership candidate. For instance, the Federation of Rental Housing Providers of Ontario gave $100,000 split evenly between five Liberal leadership candidates: Eric Hoskins, Glen Murray, Sandra Pupatello, Charles Sousa and the eventual winner, Kathleen Wynne. The landlord group also gave $20,000 each to PC leadership candidates Patrick Brown, Lisa MacLeod and Vic Fedeli, plus $25,000 to Christine Elliott.