President Bush thanked abortion opponents Monday for helping to defend his "culture of life" agenda and for pursuing "a noble cause" that he hopes will persuade other Americans to the "rightness" of their effort.

"You believe, as I do, that every human life has value, that the strong have a duty to protect the weak, and that the self-evident truths of the Declaration of Independence apply to everyone, not just to those considered healthy or wanted or convenient," Bush said by telephone to demonstrators gathered in Downtown Washington before a march to Capitol Hill. "These principles call us to defend the sick and the dying, persons with disabilities and birth defects, all who are weak and vulnerable, especially unborn children."

Monday's rally marked Sunday's 33rd anniversary of the Roe v. Wade decision that legalized abortion in 1973. The president did not call for an end to abortion rights, but identified with the aspirations of those who do.

"We're working to persuade more of our fellow Americans of the rightness of our cause. And this is a cause that appeals to the conscience of our citizens and is rooted in America's deepest principles," the president said from Manhattan, Kan., where he delivered a speech at Kansas State University. "And history tells us that with such a cause we will prevail."

The rally and march on Washington comes one day before the Senate Judiciary Committee is to vote on nominee Samuel Alito's elevation to the Supreme Court. Abortion politics and the law are central themes in the nomination, as Alito has been chosen to replace retiring Justice Sandra Day O'Connor, a swing vote on the court regarding abortion rights and other controversial topics.

Only one Senate Democrat, Ben Nelson of Nebraska, has said that he will support confirmation. Most other Democrats will oppose Alito, even though he won a unanimous well-qualified rating from the American Bar Association. Bush said Monday he didn't know if Alito would be filibustered.

"We've got legislators to write law. The judges are to interpret law. ... He is a very, very smart, capable man. When you talk to Sam Alito you think, 'smart judge.' He's written a lot of opinions, his judicial philosophy is clear and his judicial temperament is sound. That's why the American Bar Association" voted unanimously for him, Bush said.

Analysts say abortion politics explains the Democrats' hostility.

"Democratic senators can't afford to own this nominee. If they vote for Sam Alito and he drops the hammer on Roe, they might not recover," said George Washington University law professor Jonathan Turley.

Chief Justice John Roberts won confirmation in September on a vote of 78-22. Twenty-two Democrats and one independent joined all 55 Republicans in supporting Roberts' confirmation.

The perception among many opponents of Alito is that he will shift the balance of the court on abortion. But a former O'Connor law clerk said O'Connor wasn't alone in charting the court's direction on the issue.

"There's no doubt the composition of the court remained static for about 10 years. Justices Kennedy and O'Connor were together perhaps a swing vote. So it is inaccurate to say that O'Connor was the swing vote," said former clerk Andrew McBride.

McBride gave the example of 1992's Planned Parenthood v. Casey, a case in which key aspects of Roe v. Wade were upheld but states were given more freedom to impose regulations on access to abortion. Thirty-four states have since passed laws requiring parents either to be notified or to give consent when their underage daughters seek abortions.

Even with all the abortion opponents assembled in Washington and in other cities on Monday, supporters of abortion rights held their own strong showing over the weekend. A rally on Sunday focused on the Alito nomination, urging the Senate to reject him. They held a candlelight vigil in front of the court, waving signs that read: "Alito--No Justice For Women," and "Keep Abortion Legal."

Members of Congress, including Reps. Chris Smith, R-N.J., Mike Pence, R-Ind., and Melissa Hart, R-Pa., addressed the rally in Washington. They encouraged young people opposed to abortion to run for public office.

"It's time to get off the streets and into the government suites," Smith said.

While many abortion rights rallies were held on the actual day of the anniversary, abortion opponents took Monday to rally in several locations. In St. Paul, Minn., Katie Whitte braved below-freezing temperatures outside the state capitol to march for the first time against abortion.

"This year is special for me because I am a mother out of wedlock," said Whitte, 20, whose daughter is 5 months old. "I wanted to get the message out that life is important. It doesn't matter what your circumstances are."

FOX News' Major Garrett and The Associated Press contributed to this report.