Political extremist Lyndon H. LaRouche Jr. is not letting his trial on obstruction-of-justice charges impede his presidential campaigning. And it apparently is not hurting his fund-raising, either.

CBS-TV has scheduled a half-hour prime-time advertisement for LaRouche Feb. 4, from 8 to 8:30 p.m., preempting a Charlie Brown cartoon special. A LaRouche spokesman said the nationwide ad will cost "about $250,000."

This is LaRouche's fourth presidential run, and last time around television viewers were treated to 14 half-hour national campaign ads at a cost of up to $230,000 each. Most showed LaRouche sitting in front of a fireplace in his Leesburg mansion expounding on his ideas, such as that former Vice President Walter F. Mondale is a Soviet agent.

We might expect more of the same this political season, because LaRouche spokesman Dana Scanlon described the upcoming program as "action-packed."

Federal prosecutors say that LaRouche's pressure on his followers to raise money for the 1984 presidential ads led to the defrauding of many campaign contributors of hundreds of thousands of dollars. The group denies the charges, the subject of the pending criminal trial in Boston.

In addition, LaRouche and his group are under investigation by a federal grand jury in Alexandria for alleged fund-raising fraud, and a number of his associates have been indicted on similar state charges in Virginia and New York.

LaRouche is running for president in Democratic primaries in 16 states, including Maryland and Virginia, Scanlon said. "We fully intend to be virtually everywhere," she said.