Cameron Hanes is not a typical environmental advocate. An avid hunter and MAGA influencer, he recently boasted to his 1.7 million Instagram followers about killing a brown bear and turning its claw into a necklace.

But in recent weeks, Mr. Hanes has used his platform to denounce a plan by Senator Mike Lee, Republican of Utah, to sell millions of acres of federal lands as part of President Trump’s sprawling tax and spending bill.

The advocacy from Mr. Hanes — and several other conservative hunters and anglers — underscores a growing rift within the Republican Party over how to manage the nation’s public lands.

“I’m a Republican, and yes, I did vote for Trump,” Mr. Hanes said in a phone interview while hiking near his Oregon home. “But I didn’t vote for this. I didn’t vote for selling millions of acres of public land.”

Mr. Lee’s proposal would require the Bureau of Land Management to sell as much as 1.225 million acres of public property across the American West. Proponents have said the region has a severe shortage of affordable housing and that developers could build new homes on these tracts.