In a significant blow to Labor ahead of the 2016 election, the union movement believes the Liberals are "winning the referendum" on light rail and has warned that Labor's vote is "soft".
A Unions ACT briefing on recent polling, leaked to the Liberals, contains worrying signals for Labor as it prepares to sign contracts on the $800 million project.
The union polling of 1446 Canberrans was done in May but Unions ACT only released some of the results.
The full polling and some of Unions ACT's conclusions were leaked to Opposition Leader Jeremy Hanson, including a warning that the tram is a "potential vote-changer", and the "Libs are winning referendum".
Voters were asked whether they would be more or less likely to vote for a candidate that supported light rail. Forty per cent said they were less likely to vote for a pro-tram candidate; 32 per cent were more likely. Among Labor voters, 23 per cent were less likely to vote for a candidate that supported light rail.
The May polling showed 37 per cent of people would vote Labor, ahead of 32 per cent Liberals and 15 per cent Green.
But when asked whether they was a chance they might change their minds, 40 per cent said yes, with the number much higher among Labor and Green voters. Forty-eight per cent of Labor voters and 45 per cent of Greens said they might change their minds. Just 35 per cent of Liberal voters said they might change.
The union's conclusion was "Labor's vote is soft" while the "Libs vote is rusted on".
Unions ACT is running an advertising campaign saying light rail will "create 3500 new, local jobs". The figures come from a government report which predicted 3560 "gross footprint" jobs, but 1930 direct and indirect "net achievable" jobs during construction. The Ernst and Young report also warned against saying the project would "create" 3560 new jobs, saying the better word was "supports" that number.
The unions are putting flyers in letterboxes across the territory, calling voters in Mr Hanson's electorate with recorded messages encouraging them to contact Mr Hanson to support the project from next week, and advertising on buses.
Mr Hanson said the union's own commentary showed Labor was losing the light rail debate and the union movement was running a scare campaign. The union campaign was not about jobs, but about propping up Labor's vote.
"By the unions' analysis Labor voters and Green voters are prepared to desert the Labor Party and the Greens in significant numbers. Nearly 50 per cent are prepared to jump ship because of light rail," he said, claiming significant disquiet among unions about the tram.
"They are not spending thousands or perhaps tens of thousands of dollars for fun. They are doing it because they are deeply worried."
Capital Metro Minister Simon Corbell would not comment, with a spokesman referring questions to Unions ACT boss Alex White.
"If this is Unions ACT commentary on a Unions ACT poll then you'll need to talk to Unions ACT," the spokesman said.
Mr White said it was clearly "a very controversial project" but the union jobs campaign had already shifted attitudes. More recent polling, yet to be released, showing "a strong shift in public opinion towards supporting light rail."
"If Jeremy Hanson is relying on the Unions ACT polling as vindication, he should pack up his office because our poll also shows that he would lose the next election," Mr White said, pointing to a drop from 39 per cent to 32 per cent under Mr Hanson's leadership."
The jobs message was "an attitude changer for sympathetic and undecided voters".
Gungahlin is a key seat in the election, with Labor and Liberal set for a battle over the fifth seat in the electorate, which is likely to determine who forms government. But Mr Hanson said Gungahlin voters were concerned about issues well beyond the tram.
In 2012, Labor and the Liberals both won 38.9 per cent of the vote while the Greens won 10.7 per cent.