WASHINGTON, May 17 (Reuters) - The U.S. Supreme Court on Monday agreed to consider gutting the 1973 Roe v. Wade ruling that legalized abortion nationwide, taking up Mississippi's bid to revive a Republican-backed state law that bans the procedure after 15 weeks of pregnancy.

By hearing the case in their next term, which starts in October and ends in June 2022, the justices will look at whether to overturn a central part of the landmark ruling, a longstanding goal of religious conservatives.

The ruling by the conservative-majority court, expected next year, could allow states to ban abortions before a fetus is viable outside the womb, upending decades of legal precedent. Lower courts ruled against Mississippi's law.

In the Roe v. Wade decision, subsequently reaffirmed in 1992, the court said that states could not ban abortion before the viability of the fetus outside the womb, which is generally viewed by doctors as between 24 and 28 weeks. The Mississippi law would ban abortion much earlier than that. Other states have backed laws that would ban the procedure even earlier.

"Alarm bells are ringing loudly about the threat to reproductive rights. The Supreme Court just agreed to review an abortion ban that unquestionably violates nearly 50 years of Supreme Court precedent and is a test case to overturn Roe v. Wade," said Nancy Northup, president of the Center for Reproductive Rights, which is among those challenging the law.

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Mississippi Attorney General Lynn Fitch, a Republican, said she is committed to defending the law's constitutionality.

"The Mississippi legislature enacted this law consistent with the will of its constituents to promote women's health and preserve the dignity and sanctity of life. I remain committed to advocating for women and defending Mississippi's legal right to protect the unborn," Fitch said.

The Roe v. Wade ruling recognized that a constitutional right to personal privacy protects a woman's ability to obtain an abortion. The court in its 1992 decision, coming in the case Planned Parenthood of Southeastern Pennsylvania v. Casey, reaffirmed the ruling and prohibited laws that place an "undue burden" on a woman's ability to obtain an abortion.

Abortion opponents are hopeful the Supreme Court will narrow or overturn the Roe v. Wade decision. The court has a 6-3 conservative majority following the addition last year of former President Donald Trump's third appointee, Justice Amy Coney Barrett.

LOUISIANA RULING

The Supreme Court in a 5-4 June 2020 ruling struck down a Louisiana law that imposed restrictions on doctors who perform abortions. The late liberal Justice Ruth Bader was still on the court at the time, and conservative Chief Justice John Roberts voted with the court's liberal wing in the ruling. Roberts, however, made it clear that he voted that way because he felt bound by the court's 2016 ruling striking down a similar Texas law.

The 2018 Mississippi law, like others resembling it passed by Republican-led states, was enacted with full knowledge that was a direct challenge to Roe v. Wade.

After the only abortion clinic in Mississippi, Jackson Women's Health Organization, sued to block the measure, a federal judge in 2018 ruled against the state. The New Orleans-based 5th U.S. Circuit Court of Appeals in 2019 reached the same conclusion.

"States may regulate abortion procedures prior to viability so long as they do not impose an undue burden on the woman's right, but they may not ban abortions. The law at issue is a ban," 5th Circuit Judge Patrick Higginbotham wrote.

Abortion remains a divisive issue in the United States, as in many countries. Christian conservatives are among those most opposed to it. U.S. abortion rates have steadily declined since the early 1980s, reaching the lowest levels on record in recent years, according to the nonprofit Guttmacher Institute.

Jeanne Mancini, president of the anti-abortion group March for Life, said a ruling limiting abortion to early pregnancy would be in line with many other countries.

"States should be allowed to craft laws that are in line with both public opinion on this issue as well as basic human compassion, instead of the extreme policy that Roe imposed," Mancini said.

The Louisiana case ruling marked the court's first major abortion decision since Trump appointed Brett Kavanaugh in 2018 and Neil Gorsuch in 2017 as justices. Both voted in favor of Louisiana's restrictions. If Barrett were to vote on similar lines, conservatives could have a majority to curb abortion rights regardless of how Roberts votes. Trump promised during the 2016 presidential race to appoint justices who would overturn Roe v. Wade.

The Mississippi appeal had been pending at the court since June 2020. During that time, Ginsburg died and was replaced by Barrett and Trump lost his re-election bid, to be replaced by Democratic President Joe Biden, who supports abortion rights.

(Reporting by Lawrence Hurley; Editing by Will Dunham)