Twenty months ago, on a sunny January day in Washington, thousands of elated anti-abortion people gathered for a post-Inaugural celebration. The marchers, standing on the muddy Ellipse behind the White House, were quick to claim the spoils of the 1980 election victory.

One after another, their leaders introduced new senators and congressmen with the proud possessive phrase, "Here's another of 'our' new men." By noon, the Human Life Amendment had been introduced in Congress. Before the day ended anti-abortion leaders had become the first special-interest group to have an audience with the president.

It seemed almost inevitable then that a major piece of anti-abortion legislation -- a right-to-life amendment, or statute, or something -- would be won by people who put up such a display of momentum, such an impression of political power.

But last Friday, the 97th Congress prepared to recess for another election without passing a single major piece of anti-abortion legislation. More than a dozen bills were introduced: bills to define the origin of life; bills to hobble the Supreme Court; bills to change the Constitution; bills bearing the names of Helms, Hatch, Hyde, Hatfield. Not one of them has become law.

The last, a rider, attached improbably to a bill to raise the national debt ceiling, was talked to death on the Senate floor because there weren't enough votes to end the filibuster.

Somewhere along the way, the momentum of the anti-abortion movement lost its pace and the flexed political muscle lost its tone.

In part, the anti-abortion forces never displayed the unity after this early victory that they had in its pursuit. They splintered almost immediately into purists and assorted pragmatists. There was no passable legislation that won support from all factions.

Nor did the president come through as expected. On the Ellipse that day, someone held a poster declaring, "Reagan, you counted on us to win, now we're counting on you to win." But the president, for all his verbal support, never made abortion a priority.

From the Inaugural address to the current campaign speeches, the economy has been the main theme in Washington because it has been the main theme outside of Washington. More than one adviser reminded the president that people who are out of work don't want to hear about school prayer and abortion.

But what finally kept the anti-abortion forces at bay all these months was basic stuff: public opinion and political organizing.

Despite all the claims by anti-abortionists in 1980, every poll has shown that two-thirds of the Americans are against banning abortions. As Nanette Falkenberg, the head of the National Abortion Rights Action League, says, "That was always working in our favor. But the pro- choice people had gotten lazy. What happened in the last two years is that our side really did get organized."

A coalition of groups -- NARAL, Planned Parenthood, the Religious Coalition for Abortion Rights, the American Civil Liberties Union -- developed a more sophisticated strategy in and outside Congress. This session, for the first time, members of Congress were as likely to be lobbied by one side as the other, as likely to find campaign help from one side as the other, as likely to hear from abortion-rights voters as from anti-abortion voters.

"What hearing from these thousands of constitutents did," says Falkenberg, "was reinforce the idea that maybe there is something to those public- opinion polls that tell me they don't want this issue legislated."

Gradually, some members of Congress became less skittish about supporting abortion rights, others like Sens. Edward Kennedy (D-Mass.) and Max Baucus (D-Mont.) went from quiet to public support.

For election season, pro-choice groups came out of the primaries with their own list of winners. Politicians who were afraid to accept the money or even the endorsement of pro-choice groups in 1980 now come to them for volunteers and organizers.

All this doesn't mean that the abortion controversy has ended. It may never end. Sens. Jesse Helms (R-N.C.) and Orrin Hatch (R-Utah) have promised to come back with more proposals. Anti-abortion forces have shifted their focus to state legislatures, the courts, and clinics. Emotions run as high as ever.

But today there's a much greater chance that abortion will remain a personal decision -- much greater than I would have believed back on that chilling day in January.