Capitol Police at first put the crowd at between 5,000 and 8,000, but later in the afternoon that estimate had swelled to 15,000.
Those figures contrasted with police estimates of around 35,000 for marches during most of the past decade, and organizer claims in the mid-1990s of more than 100,000 participants.
As usual, marchers gathered on the Ellipse near the White House for speeches and pep talks before making the long march up Capitol Hill along Constitution Avenue.
Both ends of the march were symbolic.
Its destination, the Supreme Court, was the scene of Roe vs. Wade in 1973, when a majority of the court recognized that women have a privacy right to obtain abortions, without substantial interference from the state, during the first two trimesters of pregnancy.
The beginning venue of the march, the White House, is now the home of President George W. Bush, an avowed opponent of abortion rights who has promised to appoint judges with similar views to the federal courts, and to the Supreme Court.
Thousands of the marchers, chanting slogans and carrying small signs, broke against police barriers outside the Supreme Court Tuesday.
As it has been in the last few years, the crowd was heavily white middle class and seemed to be composed of mostly Catholic school students and their families.
Angela Gill, a 27-year-old Dallas resident seemed to be typical of many of the marchers.
A youth minister at St. Thomas Aquinas Church in Dallas, Gill said she thought many students who would have normally attended the march were scared away.
"Anthrax scares being more prevalent here than anywhere else," Gill said matter-of-factly. "I know parents are concerned."
Barely a hundred yards away, the Senate Hart Office Building had just been opened up Tuesday after being closed for months for anthrax decontamination.
The building was contaminated by an anthrax-bearing letter sent to the office of Majority Leader Tom Daschle of South Dakota, a supporter of abortion rights.
But Gill also believes Bush's election will eventually change the makeup of the Supreme Court. "I think that in time and with his help, abortion will become illegal again."
Another march participant, Stasia Zolandt Vogel, admits to being a "half-century old, plus a couple of years."
An attorney in upstate New York -- "I'm a Buffalo girl" -- and former township prosecutor, Vogel has been coming to participate in the march for the last 20 years.
Unlike Gill, she doesn't believe there will be any change in abortion until there is a massive shift in how Americans think about the issue.
"What has to change is public opinion," Vogel said. "Public opinion has to turn against abortion."
That will take some serious work and "prayer," she added.
The Rev. Paul Berghaut of St. Anthony's Parish in Falls Church, Va., was typical of the many religious participating in Tuesday's march.
Though he can't cite the cases, Berghaut said he has read that many judges are beginning to recognize the "personhood" of the fetus.
"Just our presence here is a victory of sorts," Berghaut said, looking beyond the police barriers to the Supreme Court building. "It's a softening of hard ground."
A small majority of the Supreme Court still supports abortion rights. The latest landmark ruling on abortion brought a 5-4 vote in 1999.
In that ruling, Stenberg vs. Carhart, Justice Sandra Day O'Connor, a moderate conservative, joined the court's four liberals to strike down Nebraska's ban on "partial-birth" abortions, and similar bans in three score states.
However, even though he wrote his own dissent in Stenberg, moderate Justice Anthony Kennedy indicated he still supported a woman's basic right to an abortion -- making support for abortion rights on the Supreme Court 6-3.
Those hard numbers mean Bush would have to have fill a minimum of two vacancies on the Supreme Court to change its direction on abortion -- and the two empty chairs would have to belong to justices supporting abortion rights, not the three members of the court who oppose them: Chief Justice William Rehnquist and Justices Antonin Scalia and Clarence Thomas.
Those numbers didn't stop the National Organization for Women from painting the political situation in rather stark colors before Tuesday's march.
In a statement released to the media Monday, NOW president Kim Gandy said, "Roe vs. Wade is in more danger today than at any other time in the last 29 years. George W. Bush's anti-abortion agenda could completely tip the scales of justice against women's rights. His commitment to stacking the federal courts with right-wing ideologues is frighteningly clear."
Gandy also had a clear message for the Democratic-dominated Senate, which must confirm any of Bush's judicial nominees. "You have more to fear from the women's vote than from right wing ideologues in the Bush administration," Gandy said. "Come November, supporters of reproductive rights will remember every senator who failed to oppose George Bush's effort to stack the federal courts with anti-abortion, anti-woman judges. To any senator -- Democrat or Republican -- who votes to confirm such judges: You might as well start packing your bags now."