Albuquerque, N.M., voters will cast ballots today on a 20-week abortion ban.

If passed, it would be the first municipal-level ban on abortions, according to the anti-abortion-rights group Susan B. Anthony List, which has spent $100,000 running TV ads in support of the measure.

Read more: National Abortion Fight Looms Large

The Albuquerque City Council voted 5-4 in September to put the "Pain Capable Unborn Child Ordinance" on today's ballot, which will coincide with a city councilor runoff election.

The proposed city ordinance would ban abortions after 20 weeks post-fertilization, the point at which abortion-rights opponents say a fetus is capable of feeling pain.

Read more: Anti Abortion Activists Give Children Toy Fetuses at State Fair

Twelve states now ban abortions at 20 weeks or fewer, after state-level abortion bans swept through conservative and purple states in recent years, enabled by GOP gains in state legislatures in 2010 and 2012. In each case, bans were passed by state legislatures-not as ballot initiatives.

Nebraska enacted the first 20-week abortion ban in 2010. Laws in Arizona, Idaho, and Georgia have been blocked by court cases; Arkansas' 12-week ban and North Dakota's 8-week ban are on hold pending the outcome of federal court cases.

Read more: Why Have So Many States Banned Abortions?

If Albuquerque's measure passes, it will likely be challenged by pro-abortion-rights advocates.

Under Roe v. Wade and Planned Parenthood v. Casey, abortions are legal until a fetus reaches "viability"-the point at which it could survive outside its mother's body-usually between 24 and 28 weeks. The 12 state bans all run counter to those rulings, abortion-rights advocates, legal experts, and North Dakota's Republican governor-who signed the nation's most aggressive abortion ban this year-have all said. Abortion opponents have said their goal is to overturn the Roe decision.

Sen. Lindsey Graham, R-S.C., this month introduced a federal 20-week abortion ban in the U.S. Senate.