By Richard Cowan WASHINGTON (Reuters) -A bill that would make abortion legal throughout the United States was headed toward defeat in the U.S. Senate on Wednesday, amid heavy Republican opposition. Democrats had sought to head off an impending Supreme Court opinion that is expected to overturn the nearly 50-year-old Roe v. Wade decision that established the national right to abortion, but was a protest gesture that never stood much chance of success. With at least 41 votes in opposition to the bill, Democrats were falling short of the 60 out of 100 Senate votes needed to advance their “Women’s…