In Olympic Valley, over beers at the anniversary celebration, Mr. Bullock, the Montana Democrat, and the Republicans Matt Mead and Butch Otter, the former governors of Wyoming and Idaho, said they built alliances a decade ago on wildfire costs and sage-grouse habitat that grew into friendships. They became so close, they said, that the two Republicans later refused their party’s requests to campaign against Mr. Bullock, who was governor from 2013 to 2021.

“I told them, one, he’s a friend,” Mr. Mead said. “Two, I think he’s doing a good job for the state of Montana. And three, if you want to have the kind of camaraderie that leads to cooperation and results, I can’t campaign against these guys.”

In 2014, when the three friends were part of the core membership of the Western Governors’ Association, a photo of the annual meeting featured 10 governors from both parties, representing more than half of the 19 member states. (The group also includes three territories.)

This year, only five participated in the full event. Four were Republicans: Gov. Joe Lombardo of Nevada, Gov. Brad Little of Idaho, Gov. Doug Burgum of North Dakota and Mr. Gordon of Wyoming, the outgoing chair. Ms. Lujan Grisham, who was being sworn in to succeed him, was the only Democrat who stayed at the conference for more than a couple of hours.

Mr. Newsom stopped by, but only to welcome the 300 or so attendees on the first night, most of them policy staff members, consultants and members of the organization’s youth leadership program. On an outdoor patio at the Everline Resort & Spa, Mr. Newsom and Mr. Gordon thanked each other and briefly acknowledged the association’s history of bipartisanship.