The ranking Republican and Democrat on the Senate Foreign Relations Committee voiced sharp warnings yesterday to right-wing political parties in El Salvador that any attempt to form a government without the more moderate Christian Democrats would lead to a cutoff of all aid to that country.

Committee Chairman Charles H. Percy (R-Ill.) said that "any government that does not include the Christian Democrats, which does not seek to bridge political differences in the interests of national unity and peace, will not be credible to Congress and cannot expect the support of Congress."

Percy said Salvadoran parties struggling to put together a government in the wake of elections March 28 "must be made to understand" that not only the Christian Democrats but also reforms started by the now-unseated Christian Democrat President Jose Napoleon Duarte must be included in whatever government emerges.

Even though the Christian Democrats received more votes than any other party in the elections, Percy said they could be dealt out of the new government.

The Christian Democrats received about 543,000 of the 1.5 million votes cast, but more than 800,000 votes went to five opposing right-wing parties.

Percy made his comments yesterday at a hearing on the administration policy in Latin America. The State Department's deputy assistant secretary for inter-American affairs, Steven W. Bosworth, told Percy that the administration "welcomed his statement as a powerful expression of Congress' views" and that it was "our conclusion that the political leaders" in El Salvador are aware of those views.

Sen. Claiborne Pell (R.I.), the panel's ranking Democrat, went further than Percy, saying he believes that "unless the new government moves quickly to end human rights abuses, completes the land reform program at an early date and opens negotiations with guerrilla forces," the United States "should stop all military assistance to El Salvador."

Percy, Pell and Christopher J. Dodd (D-Conn.) pressed Bosworth on whether or when negotiations might start with leftist forces in an effort to end the Salvadoran fighting.

Bosworth said the administration "opposes the concept of direct negotiations on the sharing of political power, negotiations that would occur outside the ongoing political process in El Salvador."

However, Bosworth said that "given the massive repudiation of the guerrillas," as evidenced by the big election turnout, the United States might be prepared to offer some "assistance" and "facilities" if some elements of the leftist opposition on the battlefield decide to participate in the politcal process.

Dodd told Bosworth that he did not understand what that meant and asked if the administration would support negotiations to achieve a cease-fire.

Bosworth said that "if the object of those on the extreme left was a wish to participate in the political process," the answer is "yes." Dodd told Bosworth that he sounded as though somebody "had wound you up to give rote responses."

Under questioning, Bosworth also said his department "rejects categorically" news reports that the administration is stalling on opening negotiations with Nicaragua on mutual problems. Bosworth said he expects a response to Nicaragua's most recent statement on these talks within the next "several days."