Support for lesbian, gay, bisexual and transgender rights has increased or remained steady among Mormons, Muslims, political independents, Southerners, seniors and a number of other demographic groups in the United States since 2015, a new survey found. One notable outlier: young Republicans.

Between 2015 and 2018, support for laws aimed at protecting L.G.B.T. individuals from discrimination fell by nearly 10 percentage points among Republicans under the age of 30. This was one of the key findings from a survey of more than 40,000 Americans’ views on L.G.B.T. issues that the Public Religion Research Institute released Tuesday.

The majority of young Republicans surveyed — around 63 percent — continued to support such measures. But given that 74 percent were in favor in 2015, the shift was striking, said Robert P. Jones, the chief executive of the institute, a Washington-based nonpartisan nonprofit.

“It was one of the largest and most significant drops that we saw,” Dr. Jones said.

Understanding this shift would require additional study, he said, but one hypothesis he offered was that the ranks of young Republicans are thinning, with more socially liberal individuals opting to identify as independent. “The Republican Party is becoming more ideologically pure,” he suggested.