WASHINGTON, May 3 (Reuters) - America's decades-old battle over abortion rights exploded anew on Tuesday (May 3) as the Supreme Court confirmed a draft opinion that signaled it will soon overturn the landmark 1973 Roe v Wade ruling that legalised abortion nationwide.

President Joe Biden denounced the expected move as "radical" as Democrats in Washington and in statehouses scrambled to try to find a response to defend a right that women in the United States have held for almost half a century.

Some moderate Republicans were also dismayed but social conservatives were delighted even as they voiced anger that the opinion was leaked.

The court confirmed that the draft opinion, published late on Monday by the news outlet Politico, was authentic but said it did not represent the final decision of the justices, due by the end of June.

Chief Justice John Roberts announced an investigation into how the draft - authored by Justice Samuel Alito of the court's conservative 6-3 majority - was leaked, calling it a "betrayal" of the confidentiality of the judicial process.

"This was a singular and egregious breach of that trust that is an affront to the court and the community of public servants who work here," Roberts said, pledging that the disclosure will not undermine the integrity of the court's operations.

Hundreds of people on both sides of the divide gathered outside the Supreme Court building in Washington. Supporters of abortion rights chanted "off our bodies" and "abortion is healthcare", while their opponents responded: "Pro-choice is a lie, babies never choose to die."

A ruling by the court striking down Roe would give many Republicans and religious conservatives a victory they have chased for decades.

"It's a fundamental shift in American jurisprudence," Biden said of Alito's draft, arguing that such a decision would call into question other rights including same-sex marriage, which the court recognized in 2015.

"If it becomes the law, and if what is written is what remains, it goes far beyond the concern of whether or not there is the right to choose," Biden added, referring to abortion rights.

The Roe decision recognised that the right to personal privacy under the US Constitution protects a woman's ability to terminate her pregnancy.

Biden vowed to work toward getting Congress to pass legislation codifying the Roe ruling and urged voters to back candidates in the Nov 8 congressional elections who support abortion rights.

Senate Majority Leader Chuck Schumer said the chamber will vote next week on such legislation even though a similar Democratic-backed Bill already failed this year.

Amid Republican opposition, the razor-thin Democratic majority is not enough to overcome Senate rules requiring a supermajority to advance most legislation.

Even if the new Senate vote fails, as is almost inevitable, Democrats could use it to bolster their chances in the midterm elections in which Republicans are hoping to regain control of Congress.

Nearly two-thirds of Americans said they are more likely to back candidates who support the right to abortion in the November vote, according to a Reuters/Ipsos poll conducted on Tuesday.

Democrats believe that will help incumbent Democratic senators including Mark Kelly in Arizona and Raphael Warnock in Georgia, and could hurt some incumbent Republicans, such as Senator Ron Johnson of Wisconsin.