Despite Walter F. Mondale's big crowds and the legendary 1948 victory of Harry S Truman, in the end the polls were right and the Democratic presidential nominee wrong about President Reagan's landslide reelection victory.

Still, Mondale might gain some small satisfaction from the wide discrepancies among the pollsters' predictions of the president's margin.

Reagan beat Mondale by a margin of 59 to 41 percent -- an 18-point difference -- while piling up the largest electoral vote total in the history of presidential balloting.

Pollsters regard their pre-election polls as snapshots of voter preference at a precise point in time. So it is always difficult to say, after the fact, which polls were right and which were wrong.

The Gallup Organization's findings came closest to the actual returns, showing Reagan leading Mondale 59 percent to 41 percent in the final Gallup survey completed last Friday and Saturday.

A USA Today poll published Monday showed Reagan with 60 percent of the vote and a 25-point lead over Mondale, with 5 percent of the voters still undecided. An NBC poll the same day showed Reagan with 58 percent of the vote. Since Feb. 16, both polls had consistently showed Reagan with about the same share of the vote, suggesting that few minds were changed over the course of eight months of campaigning.

"Our numbers were pretty rock-solid," said Tony Casale, USA Today's polling director. "We found a solid base of Reagan support there from the beginning. I think we did pretty well."

The last New York Times/CBS News poll showed a 58-to-37-percent margin for Reagan, once the undecided voters were divided between the candidates.

Over the final weekend, tracking polls by The Washington Post/ABC News and by the Louis Harris Organization both showed gains by Mondale.

Between Oct. 26 and Election Day, the Harris organization saw evidence of a substantial surge toward Mondale and a 12-point gap, 56 to 44 percent. But Harris still predicted "a substantial victory" for Reagan.

The Post reported in its Monday editions that its tracking polls found "indications that Walter F. Mondale may be closing the gap." The Post-ABC weekend tracking poll showed Reagan with a 14-point margin over Mondale -- down from the 18-point difference found in an earlier poll in 48 states.

The Roper Organization was the farthest from the mark, publishing a final poll last week that gave Reagan a margin of only 10 points, 52.5 to 42.5 percent, with 5 percent undecided. Asked how his organization fared, Roper Organization Chairman Burns Roper replied, "Not very well."

Roper said his poll was flawed by a problem of either methodology or data tabulation, or maybe both. "I just don't know," he said. "There is nothing I can do at this point."

The weekend tracking polls for the two candidates showed similar discrepancies. Reagan campaign officials said their poll showed the president with a 19-point lead on Sunday night, while the Mondale camp put Reagan's lead at 14 points.

The Reagan poll also showed the president with a 2-point lead in Minnesota, Mondale's home state. In the end, Minnesota was the only state Mondale carried, by a 51-to-49-percent margin.