The Democratic-led U.S. Senate is expected on Thursday to defeat a largely symbolic measure that would exempt employers such as Roman Catholic hospitals, universities and charities from a controversial White House rule requiring free birth control coverage.

Debate on the Republican proposal, introduced by Missouri Senator Roy Blunt as an amendment to an unrelated highway bill, began on Wednesday and both parties seized the opportunity to play to voting constituencies considered crucial in November’s election.

The Obama policy requires almost all employers who provide health insurance to provide coverage for women’s contraceptives without copays or deductibles. It is vehemently opposed by social conservatives and Roman Catholic bishops on moral and religious grounds.