BOSTON, DEC. 10 -- A former aide to political extremist Lyndon H. LaRouche Jr. was convicted today of plotting to obstruct a federal investigation of alleged credit card and loan fraud by LaRouche's 1984 presidential campaign.

Roy Frankhouser, who faces up to five years in prison and a $250,000 fine, was the first of 19 defendants in the fraud and conspiracy case to stand trial.

The Leesburg-based LaRouche, five of his political organizations and six of his aides are scheduled to go on trial Monday.

Three other indicted LaRouche aides have been granted later trials and three are fugitives.

The U.S. District Court jury deliberated for three hours before finding Frankhouser guilty of conspiring to obstruct justice.

He was freed on personal recognizance pending sentencing Feb. 9 and left court without commenting.

Assistant U.S. Attorney John Markham said the government was gratified by the verdict, but declined further comment because of the pending trial.

Frankhouser, 48, of Reading, Pa., a former member of the Ku Klux Klan and the American Nazi Party, was a LaRouche security consultant for seven years.

He left the organization in late 1986, about the same time the first indictment in the case was returned.

LaRouche was added as a defendant in July, charged with conspiring to obstruct the Boston grand jury investigation by ordering fund raisers to leave the country, burn records and refuse to cooperate with investigators.

LaRouche, who has accused prominent world leaders of being Soviet agents and called for a quarantine of AIDS patients, is seeking the 1988 Democratic presidential nomination.

He has run for president three times.

The 125-count indictment charging LaRouche, Frankhouser and the other defendants alleges that LaRouche followers nationwide raised millions of dollars by submitting fraudulent charges to the credit card accounts of prior LaRouche contributors and by securing loans with no intention of repaying them.

Prosecutors have said in court hearings during the last year that the money was to pay for LaRouche's 1984 campaign and the move of his national headquarters from New York City to Leesburg.

The alleged cover-up was designed to prevent the grand jury from gathering evidence against LaRouche and his inner circle, prosecutors contend.

Frankhouser was tried alone because his defense was to blame LaRouche for the alleged cover-up. Defense lawyer Owen Walker portrayed Frankhouser as a minor player in the LaRouche organization whose advice to settle with alleged fraud victims was disregarded.

But Markham and prosecution witnesses told the jury that Frankhouser advised LaRouche to send three Boston-area fund raisers abroad and to burn incriminating records. The three remain fugitives.

The prosecution and defense both told the jury that Frankhouser, who has a previous conviction for supplying dynamite used in a 1971 Michigan school bus bombing, got his job by lying to LaRouche about having high-level CIA contacts.

In addition to the Boston case, state authorities in New York and Virginia have charged nearly 30 LaRouche supporters with fraud and related offenses.

A federal grand jury in Alexandria also has been investigating LaRouche and his organizations for alleged tax violations.

LaRouche, who says he is almost penniless, has acknowledged not filing tax returns for several years.