It is a requirement that churches remain nonpolitical in order to keep their tax exempt status in the U.S.
This was always a little enforced law. Republicans made sure it was even less enforced when they became the darlings of the white Evangelicals. And the Democrats fear that a crackdown would turn their small Christian base against them. Even organizations like AARP and ACLU, while not directly endorsing candidates make their choices quite obvious by innuendo.