JONAS SAVIMBI is in town. He leads the UNITA guerrilla forces, which are pressing the Soviet-and Cuban-supported government in Angola, and he's here to win American aid. His friends, including some Reagan officials, regard him as just the kind of Third World democrat fighting communism whom the United States ought to embrace. The view that currently prevails in the administration, however, is to hold back, at least for now. Secretary of State George Shultz somewhat cooled Mr. Savimbi's Washington welcome by saying that a congressional resolution of support would be fine, but not a vote for aid.

The restraint was wise. As an Angolan leader, Mr. Savimbi has personal and nationalist credentials no less worthy and by some lights perhaps more so than those of the Marxist-oriented president now sitting in Luanda. But there are heavy costs involved for the United States in undertaking to help set him up as the ruler in Angola.

Notwithstanding his military prowess, Mr. Savimbi is a long way from Luanda: he had to call in the South African air force last year to keep Angolan forces from capturing his headquarters 700 miles to the south of the capital. An aid connection with him would make the United States a working partner of South Africa, his leading sponsor, and would torpedo the administration's attempt to convey the idea that it is serious about wanting to end apartheid. The scale of aid that would let Washington replace rather than simply join Pretoria as a Savimbi sponsor is 10 or 20 times the figures being discussed by the Angolan visitor's American friends.

Freedom-fighter imagery flows easily from those who see Mr. Savimbi as the single rightful heir to power in Angola. Unfortunately, the administration's diplomatic side has lagged in making the case for trying to bring him into a political compromise. But there is a case.

A three-purpose negotiation has been proceeding -- it limps but lives -- to bring peace to Angola, independence to Namibia and border security to South Africa. The United States is running this negotiation; despite everything, only the United States has the access to all parties to keep running it. If, however, Washington stops trying to act as a mediator in Angola's civil war and instead joins it as a military patron of one side, then forget about the whole negotiation. Forget about trying to help settle southern Africa down.

The outcome is far from certain. The administration's current strategy -- to whet Luanda's appetite for political compromise by raising the possibility of aiding Mr. Savimbi later -- carries its own risks. It represents, nonetheless, a policy more sensible than the Savimbi alternative. The courtesies due a visitor should not be allowed to distract from that central consideration.