The plan to redevelop the Qualcomm Stadium site had already provoked a lot of opposition from titans in local politics. But things got impressively worse for supporters last week when labor union leaders started bashing it.

The same goes for the mayor’s plan to expand the Convention Center and put up a special election this November. In the last 10 days, labor union opposition to the special election has solidified.

Probably not coincidentally, a steady stream of San Diego City Council members has come out against the special election, even while some maintain their support for an expanded Convention Center.

Now the special election that once seemed easy for the mayor to get on this year’s ballot is hanging by a thread, dependent on Republicans supporting it in a block and wooing Democratic Council President Myrtle Cole.

Cole is not talking.

If the special election doesn’t happen, SoccerCity is in terrible trouble. Boosters say it must happen this year for Major League Soccer to consider it.

Union leaders say they’re facing both projects with a solidarity unlike anything in the past — they are going to bat for one another and for community groups to secure specific deals and pursue higher ideals.

The bottom line is that they may have killed SoccerCity and upended the mayor’s carefully laid out plan for a November election that would have shaped his legacy and the city for decades.

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Carol Kim, political director of the Building Trades Council, a coalition of construction unions, said SoccerCity’s investors approached her group about supporting their plan to build housing, an entertainment district, large riverfront park and soccer stadium at the Qualcomm Stadium site.

They initially talked to Building Trades about constructing the stadium.

“It became evident that there were going to be hotels and many other local workers who could be impacted by the development,” Kim said.

So Kim and her colleagues decided to stand together with unions that might represent those workers.

“That’s what labor is. We stand together on these things,” Kim said. “There were some meetings that happened, and as far as we could tell, it wasn’t going to go anywhere in terms of those type of community benefits we want.”

Kim said that when San Diego State University walked away from negotiations, they decided it was not a good deal for the public and that it should be on the 2018 ballot, when more people vote.

Nick Stone, a partner in the investment group pushing SoccerCity, declined to comment on the negotiations, saying conversations were ongoing. He has said 2018 would be too late for a vote on the project.

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The mayor is at his wit’s end. He’s gone on a flurry of media appearances.

He called me. He’s as fired up as I’ve ever seen him. And for the first time, he’s very much alone on some giant proposals that would reshape the city forever if they succeed.

Most important to him right now is to get the special election scheduled for November. A City Council meeting next week could make that very hard to do if the Council doesn’t approve a budget to pay for the election.

He says he’s most perplexed about why people who normally support tax increases and want money dedicated to support homeless services are not coming aboard.

“How do you argue, ‘Yeah, it’s a crisis but we don’t want a dedicated revenue source for it — for the first time ever — until at least another year goes by?” he said. “Waiting a year makes absolutely no sense.”

Friday, his office put out an analysis.

“A new financial breakdown shows a projected revenue loss and cost impact of more than $75 million if a public vote on the tourism ballot measure is delayed for a year,” it reads. The total is a summation of the higher costs for building the expansion of the Convention Center the more time goes on and the opportunity cost of not spending on homeless and streets needs now.”

Faulconer said he went to bat for the measure and “spent a lot of political capital” bringing his Republican allies and the hotel industry along on the plan to invest in streets and homelessness.

“Big cities, when you have an opportunity, you move, man,” Faulconer said.

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Faulconer dismissed the emerging criticism from labor and its allies that there should not be a special election because of last year’s Measure L, which requires new ballot initiatives to go on general election ballots in November.

There’s an exception for special elections that the City Council decides should go before voters sooner than the next general election. The mayor says that’s an easy call here, pointing to the homelessness evident on San Diego streets and the need to provide certainty to major conventions considering long-term deals with the Convention Center.

But he brought stakeholders together too late, said Carlos Cota, the leader of the International Alliance of Theatrical Stage Employees, which has members at the Convention Center.

Cota said his union supports an expansion of the Convention Center. But the mayor only brought labor to the table in recent weeks, and there wasn’t enough time to grapple with thorny issues.

And the biggest of those is the fact that there is no longer a contract for the expansion of the Convention Center.

And thus, there’s no project labor agreement like there was before. A project labor agreement is a contract in which a labor union guarantees that a construction effort will have the workers it needs. In exchange, the contractors on the job have to hire workers through the union and pay into union benefit systems.

On May 23, Deputy City Attorney Brant Will sent a memo to the mayor and City Council communicating that the city could not put anything on the ballot guaranteeing that the city would do such an agreement. It would be illegal and a lawsuit would derail the tax increase even if voters approve it, the memo said.

The memo indicates there was a move to put something like this on the ballot. And it may help explain what provoked labor to start vociferously opposing the measure.

But it’s not as simple as that, Kim said.

Labor unions had asked City Attorney Mara Elliott whether the city could even do a project labor agreement. (A 2012 measure banned such agreements. Later, in the face of pressure from the state, the city acquiesced.)

Cota confirmed that a project labor agreement will have to happen to get labor’s support for the construction project but said he doesn’t know how the city could even provide that assurance at this point.

“We’d have to work on it and find a solution that takes time and a lot of work. You have to build trust, and labor doesn’t have complete trust in the mayor’s office,” Cota said. He’s a member of the Labor Council’s executive board.

Cota said the mayor was working hard with them when they finally did sit down to negotiate. “He was being straight with us but frankly, we ran out of time,” Cota said.

But he said the concerns about Measure L are real. And he, like Kim, pointed to a new coalition on the left of civil rights and progressive activists vowing to stick together. Alliance San Diego, for instance, was a major backer of Measure K, a 2016 measure similar to Measure L that says contests for elected citywide offices must be decided during a November general election.

It is one for all, all for one, he said.

“We’re as united as I’ve ever seen.”