Two Republican senators have urged President Reagan to halt plans to hide the MX missiles, suggesting instead that money be saved by putting the new missiles into existing silos and surrounding them with anti-ballistic missiles.

The alternate system would save $15 billion over the life of the program, as well as put 1,800 warheads in place by 1986, four years ahead of the current MX plan, said Sens. William V. Roth Jr. (Del.) and Harrison H. Schmitt (N.M.) in a letter to the president released yesterday.

The cost of the current plan "is too high, especially within the context of a balanced budget," the letter said, adding that the MX program does not have the support of the public.

The current plan is to protect the new generation of missiles on tracks in the desert and move them from place to place among a large number of empty silos, thus making it difficult for the Soviets to target them.

Reagan has said he will announce his decision later this week on the MX plan.

The plan suggested by Roth and Schmitt is the most recent of several alternate proposals senators have offered, including one from Sen. Paul Laxalt (R-Nev.).

Laxalt said yesterday on "Meet the Press" (NBC, WRC) that he hoped the president would adopt his and Sen. Jake Garn's plan to incorporate the MX missiles within the Minuteman system and back them with anti-ballistic missiles.

The Roth-Schmitt plan suggests that the missiles might be fired on warning of an attack. The missiles would orbit their warheads until confirmation of attack came; then the warheads could drop out of orbit or be picked up by the space shuttle if the attack were called off.