AUSTIN, Texas — Texas on Friday is poised to enact a ban on most abortions after 20 weeks of pregnancy, ending a bitter political fight that stirred national debate over what critics see as laws threatening the right to abortion in the United States.

The Republican majority in the Texas Senate is expected to vote Friday or early on Saturday to impose the ban on late-term abortions, and enact tough new regulations for clinics performing the procedure, and restrict administration of the so-called “abortion pill,” RU486.

Texas Governor Rick Perry, an opponent of abortion, has vowed to sign the restrictions into law.

Democrats, who opposed the measures and managed to stall them when Senator Wendy Davis staged a filibuster last month, conceded they will not be able to stop them.

Approval of the sweeping restrictions in the second most populous U.S. state, will be a major victory for the anti-abortion movement in the United States, which has been scoring small successes in conservative states to limit the basic right to abortion granted by the Supreme Court in 1973.

Twelve states have passed a version of the ban on abortion after 20 weeks, according to the Center for Reproductive Rights, a group of abortion rights activists. Supporters of the bans cite controversial research that a fetus feels pain by that stage of development. Courts blocked the laws in three states.

More than half of U.S. states have some form of requirement that abortion clinics, especially those performing late-term abortions, meet the health and safety requirements of centers performing outpatient surgery, according to the Guttmacher Institute, an abortion rights group. Three states — Missouri, Pennsylvania and Virginia — have strict standards similar to what Texas is about to approve, it said.

Planned Parenthood of Texas has said the requirements will be so onerous that all but six of the 42 abortion facilities in Texas could close. Republicans say that is an exaggeration.

Abortion rights advocates have been alarmed by the spread of state restrictions. They are expected to challenge the Texas law in court soon after it is enacted.