The presidents of 20 U.S. colleges and universities, many of whom have been at the center of emotional campus battles over investments in South Africa, have written to the Senate advocating tough sanctions against South Africa to protest its policy of apartheid, or racial separation.

"Because of its flagrant injustices, apartheid is condemned by Americans everywhere," the letter says. "Its pervasive racial discrimination evokes a painful reminder of our own history and spurs Americans to make common cause with the oppressed."

The letter was made public yesterday, the eve of an expected Senate vote on sanctions legislation. Senate Majority Leader Robert J. Dole (R-Kan.) suggested yesterday that he will try to end a filibuster and bring the issue to a vote this afternoon.

The university presidents who signed the letter represent some of the nation's largest and best-known institutions, including the entire Ivy League and Stanford, Emory and Rutgers universities.

The presidents stressed that they were writing as individuals, not in their official capacities. They added, however, that their views were honed after years of dealing with university investments and increasing student demands to divest stocks of firms that do business with South Africa.

The unusual joint letter on a pressing foreign policy topic was conceived by Harvard President Derek Bok, who has resisted divestment pressure at Harvard for 13 years but who came out in favor of congressionally backed sanctions in May. The letter does not mention a specific sanctions bill, but makes a general statement in favor of tough sanctions directed at the Pretoria government.