Expanded access to emergency contraception is among the objections to President Obama's birth control coverage rule, which requires most employers to cover the pill or pills in their health plans.
"Plan B" works to prevent pregnancy in the days after unprotected sex by stopping ovulation and inhibiting the movement of sperm.
For years, the Food and Drug Administration (FDA) has also said that the drug might prevent a fertilized egg from attaching to the uterus for an outcome some consider equal to abortion.
Medical authorities have called this understanding outdated and scientifically incorrect, and cite clinical studies showing Plan B has no effect on fertilized eggs before or after implantation to the womb.
The debate grew louder when the Obama administration released its birth control coverage rules under the healthcare law.
The new policy requires most employers to cover a range of birth-control methods, including the so-called "morning-after pill," in their health plans without cost-sharing.
More than 100 plaintiffs have now filed suit to block the mandate on the grounds of their religious objections to abortion.
Murray is a staunch supporter of the Obama policy.
"Improving access to birth control is good health policy and good economic policy. It will mean healthier women, healthier children and healthier families. It will save money for businesses and consumers," Murray wrote in a February op-ed with Sens. Barbara BoxerBarbara BoxerAnother day, another dollar for retirement advice rip-offs Carly Fiorina 'certainly looking at' Virginia Senate run Top Obama adviser signs with Hollywood talent agency: report MORE (D-Calif.) and Jeanne Shaheen Jeanne ShaheenRussian interference looms over European elections Restore funding to United Nations Population Fund Senators urge Tillerson to meet with Russian opposition activists MORE (D-N.H.).