As pro-choice demonstrators planned a candlelight vigil outside the supreme court in Washington, all six Democratic presidential contenders used their first joint appearance to pledge to defend abortion rights. But 50,000 anti-abortion protesters held a "march for life" addressed by President George Bush.

The debate has been sharpened by the prospect of a Republican White House and both houses of Congress overturning the pro-choice majority on the supreme court.

At least one supreme court judge is expected to retire this year and it is likely that the president will appoint a replacement who is opposed to abortion.

"The march for life upholds the self-evident truth of that declaration - that all are created equal and given the unalienable rights of life, liberty and the pursuit of happiness," Mr Bush said by telephone from St Louis, Missouri.

The president, who proclaimed last Sunday as a national sanctity of human life day, said America "must protect the lives of innocent children waiting to be born".

In 1973 the supreme court passed a ruling recognising abortion as a constitutional right, in the landmark case of Roe v Wade. But Mr Bush is committed by his party's platform in the 2000 presidential election to push through a constitutional amendment outlawing abortion.

Kim Gandy, president of the National Organisation for Women, said her group's focus would be on maintaining the current supreme court balance and ensuring that "we will not be the generation that both won and lost reproductive rights in our lifetime."

On Tuesday night, the six Democratic presidential hopefuls, all men, attended a dinner in Washington hosted by a pro-choice organisation and stressed their concern. "The freedom to choose has never been more in peril than it is today," congressman Richard Gephardt of Missouri said.

With attention focusing on the next presidential election, pro-choice advocates hope to push the issue up the political agenda. "Now the president's views and record are coming out of the shadows," said Kate Michelman, president of Naral Pro-Choice America. "It will be an issue in 2004."

A Washington Post/ABC poll had mixed results for both sides. Clear majorities exist for legalised abortions in all or most cases, including those that would save a woman's life or health, in cases of incest and rape and if the baby is physically impaired. But 57% say it should be illegal to end unwanted pregnancies, and even more oppose partial-birth abortions and those in pregnancies over six months.

Abortions are becoming less common in the US, particularly among teenagers, in part because of better contraception. The overall abortion rate for women of childbearing age fell from 2.4% in 1994 to 2.1% in 2000, said the non-profit Alan Guttmacher Institute.