There was a critical omission in a series of four stories The Post published last week about a trial in St. Louis involving Sen. Thomas Eagleton. There should have been a fifth story.

The series began on Monday with an article about Eagleton's reelection campaign. Eagleton was making few appearances because of a federal court trial of his niece, Elizabeth Weigand, and her former attorney, Stephen E. Poludniak.

Weigand and Poludniak were accused of attempted extortion. They were charged with threatening to release what they described as damaging material about Eagleton unless he purchased for $220,000 Weigand's 6.5 percent interest in a family-owned pipe fittings company.

The threat was to release the material to the St. Louis Globe-Democrat and to the campaign aides of Eagleton's opponent. The nature of the material was not revealed at that point, but the jurors had been informed that it was of a tragic nature and involved Key West, Fla.

On Wednesday there was a second Post story. It reported that Poludniak said Weigand could reveal information harmful to the senator.

Thursday The Post's third story said the senator had read to the jury a handwritten note from his niece accusing hom of a homosexual affair in Key West and both business and election irregularities. Eagleton said the accusations were "absolutely and totally false." In the same story, the niece's attorney was reported to have told the jury that the homosexual allegation was "just gossip" and wasn't "worth a thing." That was the first hint from the defendants that something was badly wrong in the accusations against Eagleton.

There was no Post story on Friday, and on Saturday a final article about the trial appeared. It reported that the niece and her former attorney, the defendants, had been convicted of conspiring to extort money from Eagleton. They face seven years in prison and fines of up to $10,000.

The Post's coverage had an obvious gap. What had happened in the courtroom on Thursday that should have been published in Friday's Post? Thursday was the day when the defendants were to testify.

Much happened, and it was reported in both St. Louis newspapers but not in The Post. Poludniak's lawyer told the jury that his client "will tell you he lied. . . . He made it up. He was blowing smoke. . . . There was nothing to this gossip."

In his own testimony, Poludniak said the allegations he and Weigand had made against Eagleton were "slanderous," libelous" and false.

Poludniak said he had written the accusatory notes and Weigand had copied them on the day they had attempted to force Eagleton to come through with the $220,000.

Poludniak also said he had told Weigand that the action they were taking was legal.

Weigand took the stand and testified that the allegations against the senator were "wild, preposterous and ridiculous" but that she had relied on Poludniak's legal advice in using them against her uncle.

The defendants' testimony and the jury's decision vindicated the senator.

What happened to The Washington Post's story on Thursday's courtroom revelations? It simply did not get into the paper. It was provided by one of the wire services, was edited and prepared for publication. Inadvertently, it did not get published. The omission left out vital information about a prominent senator in the city where Eagleton spends most of his professional life. And it left out the very words of his accusers that would have documented the senator's denials.

It should not have happened.