The Senate Defense Appropriations Subcommittee yesterday voted to spend $3.9 million to build test models of a modernized World War II fighter that the Joint Chiefs of Staff do not want.

If the rest of Congress goes along the plan is to appropriate another $6.7 million next year to finance testing and building of a modified P-51 fighter called the Enforcer.

David Lindsay, a Saraosta, Fla., newspaper publisher who developed the Enforcer, has been trying for seven years to get the Air Force to broaden its previous tests of the plane, arguing that it has not had a fair trial.

In hearings this March by the Senate Armed Services Tactical Air Subcommittee, Lt. Gen. Alton D. Slay, Air Force research chief, testified that "there is no need for a flight test of the system" and that "there is no place in the Air Force inventory for an aircraft such as Enforcer."

Slay, under questioning by Sen. Barry Goldwater (R-Ariz.), agreed it would be a waste of taxpayers' money to build test versions of the Enforcer.

In a followup letter to the subcommittee, Gen. George S. Brown, chairman of the Joint Chiefs of Staff, said "there is no requirement for the Enforcer." He added that the judgment by the chiefs "has been supported by an extensive review of the Enforcer proposal . . ."

Lindsay predicted in a telephone interview last night that even if the rest of Congress endorses the subcommittee's action, the Air Force will resist going forward on grounds it cannot award such a research contract without open bidding.