If it could put it simply - the more Liberals adopt Leftist political views and the more those views dominate the political elite and infiltrate a denomination’s theology, distinguishing the denomination as an arm of that political elite, the denomination enters and continues decline.
It is more than mere political philosophy though.
“The keys to the kingdoms of the earth” is a universal phrase - passing through all eras of time, that represents secular power & government.
In His confrontation with Satan, Satan offered Yeshua “the keys to all the kingdoms of the world”. Christ replied that they were not part of His kingdom. One only has to read and believe the Book of Revelation to understand that fact does not change throughout human history, until He, Himself, brings His kingdom to earth.
It’s of no small significance that the pro-birth control and pro-abortion denominations are declining.