Liberal Democrats from both houses of Congress asked the Reagan administration yesterday to pursue a "negotiated political solution" to the guerrilla war in El Salvador in order to head off another Vietnam-like U.S. military involvement.

At a news conference, four lawmakers announced their introduction of a joint resolution calling on the administration to back "unconditional negotiations" between the Salvadoran government and insurgent forces.

The legislators, Sens. Christopher J. Dodd (Conn.) and Paul E. Tsongas (Mass.) and Reps. Michael D. Barnes (Md.) and Stephen J. Solarz (N.Y.), conceded that it is far from certain that the opposing forces in El Salvador could work out their differences peacefully even with U.S. support in doing so.

However, they contended it is certain that the current U.S. approach, involving military aid to the Salvardoran junta and sponsorship of next month's election, will fail to resolve the bloody strife.

"The policy of escalation and pursuit of military victory can only strengthen the extremes at the expense of the center and continue the process of radicalization and polarization of political forces," the lawmakers said in a joint statement.

The legislators depicted centrist political elements on both sides, including Salvadoran President Jose Napoleon Duarte and leaders of the opposition Democratic Revolutionary Front, as ready for negotiations. Hard-line military elements on the two sides were described as likely to crush any chance for a political solution if the present trends continue.

Dodd said the situation in El Salvador, including "lights at the end of the tunnel, free-fire zones, destroying villages in order to save them," is reminiscent of Vietnam. In the 1960s, he said, Congress failed to debate the issues properly before the United States became too deeply involved to consider alternatives.

Tsongas added that "El Salvador cannot be another Vietnam because you have in Congress a large number of individuals who grew up politically during the Vietnam era, and there is simply no way we're going to sit back and let Vietnam be repeated."

State Department spokesman Dean Fischer, asked about the joint resolution, reiterated the administration view that to allow insurgent forces to gain a share of power through negotiations would be "a usurpation of the right of the Salvadoran people to determine the nature of their own government."