The Reagan administration's plan to step up military aid to El Salvador's government emerged from a House committee unscathed yesterday despite determined efforts by Democrats to cut it back and to put it under new restrictions.

Four hours of often angry debate in the Foreign Affairs Committee ended with votes approving a $50 million increase in military assistance and defeat of a Democratic attempt to force the Salvadoran government to negotiate with leftist guerrillas.

It was Congress' first extended debate on El Salvador since the March election gave right-wing parties substantial control of the government and it turned largely on conflicting interpretations of what the election meant.

Republicans generally called the election a mark of progress, noting the large turnout of Salvadoran voters, while many Democrats called it a defeat for the U.S. policy of attempting to strengthen moderate forces in that Central American country.

Administration officials later expressed pleasure with the committee's markup of this year's foreign aid authorization. The day before they had been disappointed when two House subcommittees drastically reduced the amount of economic aid El Salvador will receive under the new Caribbean Basin plan.

In the foreign aid bill, the administration is seeking an increase of $74 million in military assistance for Latin American governments as part of a plan to strengthen resistance to guerrilla movements in that region. About two-thirds of that increase, $50 million, is earmarked for a single country, El Salvador.

The major Democratic assault, a motion by Rep. Gerry E. Studds (D-Mass.) to eliminate the increase, failed on a vote of 21 to 13 as Chairman Clement J. Zablocki (D-Wis.) and several other Democrats supported the administration.

Doubling the amount of aid for a Salvadoran government now controlled by right-wing forces would be "to say we are pleased" with the direction of politics there, Studds said.

Republicans argued that ending or sharply reducing aid to that country would be the end of its efforts to resist a guerrilla insurrection. "This would write off El Salvador," contended Rep. William S. Broomfield (R-Mich.), the ranking Republican. "It would only help the guerrillas."

The committee's Republicans, joined by several Democrats, also beat back an effort to make it more difficult for the president to send any military aid to El Salvador by tightening a provision first inserted last year.

That law said that to certify El Salvador as eligible for aid the president first had to make a formal finding that human rights abuses in that country were diminishing. Reagan certified that early this year, irritating Democrats who said there had been no improvement.

Rep. Stephen J. Solarz (D-N.Y.) yesterday sought to tighten that with an amendment prohibiting aid unless the Salvadoran government agrees to begin unconditional negotiations with guerrillas to end the fighting and arrange a political settlement. Both the administration and the Salvadoran government object to any requirement for unconditional negotiations.

The committee wound up voting down the Solarz amendment and accepting a substitute from Rep. Benjamin A. Gilman (R-N.Y.) requiring the government to engage in a "dialogue" with the guerrilla parties unless those opposition forces are unwilling to talk.