The Justice Department suddenly dropped its prosecution of New Jersey rice merchant Grover Connell yesterday on grounds that key witness Tongsun Park has changed his story.

The decision was challenged immediately by a former lawyer for the parallel House investigation of South Koream influence-buying. Jeffrey Harris, who headed the House inquiry into Park's rice dealings, said that even without Park's testimony "it is inconceivable to me that there is not enough evidence to bring this case to trial."

He also asked why, if Park was such a key witness Justice sought a perjury indictment against Connell without ever taking Park before an American grand jury.

William Hundley, Park's lawyer, also said yesterday, "If they're blaming this [dropping the case] on Park, it's not fair." He said Park has changed nothing but the emphasis of the story he first gave Justice prosecutors in Seoul more than a year ago.

Hundley also noted that he was surprised by the Connell indictment initially because his client's story had never been checked before an American grand jury.

Park's testimony about Connell in Seoul was under oath, but was given after Hundley had left Korea and was not checked by a polygraph, officials said yesterday.

Connell was indicted a year ago on six counts of racketeering, conspiracy and perjury. The department dropped four counts without explanation in January.

The remaining two perjury counts were dropped yesterday as the trial was to start in U.S. District Court in Newark. Connell had been accused of lying to a federal grand jury when he said he didn't know Park was the recipient of more than a million dollars in commissions his firm paid on rice sales to South Korea from 1972 to 1974.

Justice Department spokesman Terrence Adamson issued a statement saying the case was dropped because Park now says he tried to keep the real arrangement secret from Connell. Connell Rice and Sugar Co. of Westfield, N.J., paid commissions during the period to Daihon Nongsan of Seoul, but the Korean company was only a front for Park in the transaction, according to public testimony.

Harris, who acted for the House Committee on Standards of Official Conduct in questioning Park, Connell, and others involved with the rice commission arrangement, said, "The Justice Department's filing of an indictment in a major case without putting the main witness before the grand jury can only be described as irresponsible and incompetent."

"Further, it's my opinion that there is sufficient evidence in this case, other than Parkhs testimony, to take it to trial."

John Kotelly, the Justice Prosecutor who was to try the case, said yesterday that the original indictment was based on "what we thought was competent evidence at the time . . . Without Park sticking to his story the corroborating testimony disappears. It could not stand by itself."

Both Kotelly and Adamson said it is department policy to ask for dismissal of a case that no longer meets a standard of good conscience.

Kotelly was called in to try the case after other attorneys prepared it for indictment. He said yesterday he would not "second-guess" the validity of the original six-count indictment. Other department attorneys, however, reported concern that the Connell indictment was overdrawn.

Connell could not be reached for comment yesterday.

The dismissal of his case is the latest in a series of setbacks for the long running Korean investigation. All counts in the department's case against former House member Otto E. Passman (D-La.) were rejected by a jury of the longtime congressman's constituents this month.

Only former member Richard T. Hanna (D-Calif.) has served jail time in the scandal. He pleaded guilty without going to trial.

Last week a federal grand jury indicted a third former congressman Nick Galifianakis (D-N.C.), saying he lied about $10,000 in cash he received from Park in 1972.