DENVER (AP) — Two years after jumping into a fight with the Trump administration over public lands, the U.S. outdoor industry is turning up the political pressure — though its impact is difficult to measure.

Thousands of manufacturers and retailers gathered in Denver starting Wednesday for the annual Outdoor Retailer and Snow Show, and some of the biggest names vowed to keep pushing to preserve public lands.

"We will always — this is really core to who we are," said Corley Kenna, a spokeswoman for Patagonia, the brashest political fighter among the industry's major players.

The Outdoor Industry Association and some big retailers, including Patagonia, Columbia Sportswear, REI and The North Face, have campaigned together and on their own to protect public lands.

Some of their forays are unobtrusive get-out-the-vote campaigns, lobbying for national parks funding and email blasts to customers about public lands news.

But Patagonia took the unusual step of endorsing U.S. Senate candidates in November's election. The company publicly accused President Donald Trump of stealing public lands and sued his administration.

The campaigns made headlines and energized the industry, but it's hard to measure the effect on voters and policymakers.

Now, show organizers have made climate change and sustainable manufacturing a priority and announced the formation of the Outdoor Business Climate Partnership to lobby for state and federal climate policies.

Issue campaigns can be effective if they are done well and if voters are receptive to the message, said Josh Kalla, an assistant professor of political science at Yale University.

"You do tend to see issue advertising does change public opinion to a much greater extent than candidate advertising," he said.

But neither the Outdoor Industry Association nor the big companies have done the kind of polls and surveys that would show that.

The Senate candidates that Patagonia endorsed in tight races — Democrats Jon Tester in Montana and Jacky Rosen in Nevada — both won.

"I would like to think that we played a part in that because we were motivating our own community to get out and vote," Kenna said.

Public lands were an issue in the Nevada election last year, said Dave Damore, a political science professor at the University of Nevada, Las Vegas. But he stopped short of saying that helped Rosen.

Montana State University political scientist Dave Parker said polling in his state showed public lands were not a significant factor in Tester's victory.

At minimum, the big retailers have pushed public lands into the spotlight, said Kayje Booker of the Montana Wilderness Association.

"These national brands have a reach that nonprofit advocates could only dream of, and they have been using that megaphone in a really effective way to remind people of this issue," she said.

Organizers of the winter outdoor show, which is drawing about 950 exhibitors and 25,000 attendees over three days, are preparing to fight another potential government shutdown to protect national parks, which were left understaffed and underprotected during the 35-day shutdown that just ended.

Columbia weighed in on the previous shutdown with a full-page ad in the Washington Post that mimicked Trump's campaign slogan: "Make America's parks open again."

The outdoor industry, which calculates its annual sales at $184.5 billion, was once reluctant to get involved in high-profile advocacy.

"We would like nothing more than to be thinking about a great hike instead of thinking about politicians," said Peter Bragdon, executive vice president of Columbia Sportswear.

Some companies and industry groups say they have always openly advocated for public lands and recreation, but Trump's election brought new players and energy to the fight.

"I would say the thing that's changed is when President Trump was elected, some of the threats to our public lands became more high profile," said Amy Roberts, executive director of the Outdoor Industry Association.

The industry began to stir in February 2017, when Utah lawmakers asked Trump to repeal the newly designated Bears Ears National Monument. Thirty outdoor companies objected, and the Outdoor Retailer Show announced it would move from its longtime home in Salt Lake City to Denver.

Things heated up in December 2017 when Trump reduced the size of Bears Ears and Grand Staircase-Escalante national monuments. Patagonia sued and declared on its website, "The President Stole Your Land."

Interior Department spokeswoman Faith Vander Voort defended the administration's record, saying it had expanded access to public lands and revived their use for ranching, mining, logging, drilling and other commercial purposes as well as recreation.

Although Trump removed land from national monuments in Utah, it still belongs to the federal government, Vander Voort said in an email Wednesday.

"Organizations like Patagonia knew this, but they chose instead to play fast and loose with the facts," she said.

Executives at REI, the giant outdoor-gear cooperative with 17 million members, said it's important to avoid casting public lands as a partisan fight. That would be a disservice to everyone who has ever fought for public lands, said Alex Thompson, an REI vice president.

"We're trying to balance being really engaged and really clear ... while also not falling into the trap of fueling unhelpful divides," he said.

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Follow Dan Elliott at http://twitter.com/DanElliottAP .