WASHINGTON (AP) _ Four senators canceled a planned trip to the Soviet Union Monday after a Senate aide was refused permission to accompany them because of an article he wrote about Soviet involvement in Afghanistan.

''As a matter of principle, we felt we had no choice but to cancel because we cannot allow the Soviet Union to engage in a form of manipulation or censorship of a Senate delegation,'' said Sen. William Cohen, R-Maine, who was to have led the group.

Cohen and Sens. Carl Levin, D-Mich.; Joseph Biden, D-Del., and Warren Rudman, R-N.H., had been planing to leave Monday for a visit at the invitation of the Soviet Academy of Sciences to discuss various issues including arms control, chemical weapons, and U.S.-Soviet relations.

Soviet officials told the delegation Friday that Senate Foreign Relations Committee staffer John Ritch III had been denied a visa for the trip because of a report he filed last year detailing alleged Soviet brutality in Afghanistan. Cohen said he gave the Soviets the weekend to change their minds, and canceled the trip when they declined to do so.

''Under no circumstances should the Soviet Union be allowed to pick and choose who of (Senate) staff should be brought into that country,'' Cohen said.

''There's a principle that is important for us, and that is that we can't allow the Soviet Union or any other country to dictate to us who will be on a congressional delegation,'' Levin said. ''They have a right to be unreasonable about who can enter their country. We have a right to choose not to go on this trip.''

Rudman called the refusal to grant Ritch a visa ''just perfectly outrageous conduct'' which does not jibe with Soviet leader Mikhail Gorbachev's statements that he hopes to warm up relations with the United States.

''The way to warm things up is not to put more ice in the pond and he has done just that,'' Rudman said.

Biden called the Soviets' decision to ''erect this barrier a discouraging signal of current Soviet attitudes.''

Ritch, who was to have served as Biden's assistant on the trip, had visited Afghanistan last year at the end of a Senate trip to the Soviet Union. He provided the Foreign Relations Committee with a 55-page report titled ''Hidden War: The Struggle for Afghanistan.''