The Liberal Party of Canada temporarily expanded its campaign headquarters during the federal election last year by renting office space from Canada 2020, a not-for-profit organization founded by long-time partisans with close personal ties to Prime Minister Justin Trudeau.

According to documents filed with Elections Canada, the Liberals subleased office space from Canada 2020 at 35 O'Connor St., in downtown Ottawa — now occupied by the think-tank — from Aug. 13 to Oct. 19, 2015, so that the party could set up a "volunteer hub" that was up and running 12 hours a day.

The documents show the Liberals paid a total of $13,833 to Canada 2020 over that period.

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There is nothing in the rules that would prohibit the arrangement, as both the Liberal party and Canada 2020 say the agreement involved paying full market rent.

But it is another example of the close connections between the Liberals and Canada 2020, an organization that styles itself as an independent source of progressive ideas and policies but whose people are also intertwined with the governing party and the prime minister.

NDP Leader Tom Mulcair cited the arrangement Wednesday during a question period dominated by the cash-for-access fundraising controversy that has dogged the Liberal government for months.

He described it as "evidence" that the think tank is "simply a wing of the Liberal Party of Canada."

"Access to the prime minister shouldn't be based on financial donations to the Liberal party, or Liberal organizations," Mulcair told the House.

Government House leader Bardish Chagger responded with a similar message to the one Trudeau delivered Tuesday when pressed on the $1,500-per-ticket fundraisers: the events are about working for the middle class, she said, provoking much mockery from the opposition benches.

"Just imagine the middle-class problems the Liberals try to solve over champagne and caviar," said Conservative interim leader Rona Ambrose.

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The think tank is conscious of the optics.

Canada 2020 has recently been trying to insulate itself from the cash-for-access inferno by bringing in new policies related to lobbying.

The close relationship also came up during the U.S. presidential election campaign, when Wikileaks published purportedly hacked emails that suggested Hillary Clinton's entourage was annoyed the Liberal party had used a Canada 2020 event she appeared at in 2014 as a fundraising opportunity.

The costs associated with the work the volunteer hub did for local campaigns across the country — including rent, meals, telephones, computers and telecommunications services — were listed as part of the "Liberal Party Riding Services Package" the central campaign sells to the local campaigns for individual candidates nationwide.

Twenty-five per cent of the total amount spent on rent, or $3,458, was billed to the local campaigns, because the volunteers were making calls for local candidates from 5 p.m. to 8 p.m. every day.

Braeden Caley, a party spokesman, wrote in an email Tuesday that the Liberal Party of Canada chose 35 O'Connor St. because of its "central location and close proximity" to its own national office at 350 Albert St., also in downtown Ottawa, which he said was not big enough to accommodate the "strong volunteer response" from the area.

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"The Liberal campaign was the sole occupant of the space," Caley wrote.

He said the volunteer hub moved back into the Liberal party national headquarters after the campaign was over.

Alex Paterson, a spokesman for Canada 2020, wrote in an email Tuesday that the organization rented the space to the Liberal party because "the space was available to rent."

He said Canada 2020 did not begin using the space until earlier this year, when it relocated from its previous location on Dalhousie Street, in the Lowertown neighbourhood just east of downtown, and that no one associated with the organization was based at the Liberal volunteer hub.

Neither Paterson nor Caley would provide a copy of the rental agreement signed between Canada 2020 and the Liberal Party of Canada.

That paperwork is included with the supporting documentation for the "Liberal Party Riding Services Package" provided to Elections Canada, but it is six centimetres tall and illegible, with an access-to-information request necessary to see the full-sized version.

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As The Canadian Press reported last month, Canada 2020 recently introduced a new donor agreement that must be signed by any company or group that gives it money, making it clear that the donation will not buy access to the prime minister, his cabinet ministers who anyone else who attends their events.

It has also recently implemented a new policy prohibiting active lobbyists from sitting on its board of directors.

Tom Pitfield, who is president of Canada 2020, was the chief digital strategist for the 2015 Liberal election campaign.

He is also a childhood friend of Trudeau's and is married to Anna Gainey, the Liberal Party of Canada president.

Canada 2020 was founded 10 years ago, when the Conservatives were in power, by Pitfield and three other longtime Liberals: Tim Barber, Susan Smith and Eugene Lang.

Smith and Barber are both principals at the lobbying firm Bluesky Strategy Group, which used to be located in the same Dalhousie Street building that housed Canada 2020, and now occupies a space at 35 O'Connor St., one floor below where Canada 2020 is now.

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Smith, a registered lobbyist, recently left the board of directors at Canada 2020 in accordance with the new policy.

Barber remains on the board, but Lang has not been involved since 2013.