Two dozen liberal-to-moderate House Democrats have warned Soviet leader Mikhail Gorbachev in an unusual private letter that the Soviet Union risks "serious consequences for the future of the arms control process" unless it complies with existing treaties.

Among the signers are three key figures in national-security policy in the Democratic-controlled House: Foreign Affairs Committee Chairman Dante B. Fascell (D-Fla.), Armed Services Committee Chairman Les Aspin (D-Wis.) and Joseph P. Addabbo (D-N.Y.), chairman of the Appropriations subcommittee on defense.

The letter contained an implicit warning that the Soviets face trouble from arms control advocates and critics of the process unless they abide by existing accords, including the 1972 treaty on antiballistic missiles.

Among the signers are some of the House's leading arms control advocates.

"The purpose of the letter is not to sabotage the arms control process but to save it," said Rep. Stephen J. Solarz (D-N.Y.), an organizer of the letter project.

"As people who believe arms control is a moral and political imperative, we are determined to let the Soviet Union know that it will not be possible to muster the necessary support in the Congress and the country for new arms control agreements unless existing treaties are respected," Solarz said in a statement about the letter.

The letter expressed concern that construction of a radar system near Krasnoyarsk in Siberia would violate the ABM treaty when the system becomes operational, a criticism previously voiced by the Reagan administration and many conservative lawmakers. The administration has officially protested to the Soviets about construction of the installation.

"If this problem is not resolved in a satisfactory manner, it will have serious consequences for the future of the arms control process," the letter said.

"Not only will additional arms control efforts to strengthen the ABM treaty, regarding space-based and other systems, become much more difficult, but continued substantive and political support for the ABM treaty itself in the Congress and among the American people will be eroded," the letter continued.

In referring to space-based weapons, the lawmakers hit on a sensitive point for the Soviets, who strongly oppose the Reagan administration's Strategic Defense Initiative anti-missile defense plans and want it included in any pact that emerges from the current arms talks in Geneva.