The state’s largest labor union, the Colorado AFL-CIO, is not endorsing incumbent Democratic U.S. Sen. Michael Bennet for re-election, showing just how much the issue of trade could play in the 2016 elections.

The lack of an endorsement has also allowed a third party candidate challenging Bennet to capitalize on the move.

The Colorado AFL-CIO represents more than 300,000 union members and families in the state. Over the weekend, its political committee met after interviewing candidates in the closely watched Colorado U.S. Senate race.

One issue dominated the discussion when it came to endorsements: Trade.

“The Trans-Pacific Partnership weighed heavily on our affiliates’ decision to remain neutral in the U.S. Senate race,” said Sam Gilchrist, director of the Colorado AFL-CIO, in a statement. “We deeply respect Senator Bennet, and all he has done for Colorado’s working families. However, the affiliates present at our convention could not reach the two-thirds majority needed to endorse.”

Bennet has not yet taken a public position on TPP.

But Bennet voted on a measure giving Obama the authority to “fast track” negotiations for it and other global trade agreements, and Bennet has drawn fire from the AFL-CIO for his stance on trade before. Democratic presidential candidates Hillary Clinton and Bernie Sanders have come out against TPP.

That the state’s largest labor group took a pass on Bennet this year isn’t to say he’s lost union support in Colorado or nationally. At least 10 organized labor groups from the Sheet Metal Workers to Air Traffic Controllers to the Rural Letter Carriers Association have contributed to his campaign.

“Michael’s proud to be supported by a majority of union members across Colorado and in the Senate he’ll continue fighting for middle-class families and working to get things done for Colorado,” says his campaign spokeswoman Alyssa Roberts.

The Trans-Pacific Partnership is a trade agreement backed by President Barack Obama among some countries that corner the Pacific Ocean and is geared toward reducing trade barriers among them.

“But it will do a lot of other things, too,” writes Vox.com in an explanation of the deal. “The agreement could require countries to adopt stricter labor and environmental rules, provide stronger legal protections to drug companies, lengthen the term of copyright protection, give foreign investors a new way to challenge countries’ laws and regulations, and much more.”

The Colorado AFL-CIO, which endorsed Bennet in 2010, is angry about the trade agreement.

“The trade deal as a whole is very concerning for us,” Gilchrist told The Colorado Independent in an interview, adding that it should have had more public input. “[Bennet] voting for fast track allowed that process to be expedited and that’s where our concern was,” he said.

Gilchrist added, however, that he felt Bennet is being thoughtful about where he’ll ultimately come down on the deal. Being on the top of the ticket, though, “he bore the brunt of our delegates anger on trade,” Gilchrist said.

Despite the labor group’s neutrality in the Colorado U.S. Senate race, the state’s Green Party candidate, Arn Menconi, who was interviewed by the AFL-CIO, says it’s a plus for his campaign.

“This is great for me to hear that they’ve taken a pass on this because it shows that the unions have realized that the Democratic Party is not fighting for unions,” Menconi says. “We’ve seen that steady decline.”

Once one of the more powerful forces in Democratic politics, organized labor is seeing its electoral might challenged by environmental groups with big money and organizing efforts at the national level.

A recent New York Times story detailed a rift between the two interest groups over turnout efforts, which has caused headaches for the Democratic Party during this election cycle.

The AFL-CIO extended an offer to all candidates to respond to their questionnaire and give an interview for a potential endorsement, Gilchrist said. No Republicans in the U.S. Senate race accepted that invitation.

The group will do another round of endorsements after the June primaries.

Photo credit: AFL-CIO America’s Union, Creative Commons, Flickr.