The Reagan administration has encountered its first notable political backlash on the issue of El Salvador, which two months ago looked like a good place to "draw the line" against communist expansion.

While the White House has made much of the overwhelming support of incoming mail on the president's economic recovery program, it has not broadcast the fact that the El Salvador mail, which it ships over to the State Department, is running 10 to 1 against the administration's new emphasis on military aid and advisers for the impoverished Central American country.

The signals are fairly clear: the public is not enthusiastic about the possibilities of U.S. involvement in a foreign guerrilla war.

A Gallup Poll made public today showed that Americans who are familiar with the U.S. role in El Salvador are sharply divided over it, with about half approving of the president's handling of the situation and half disapproving. The same sample also was equally split over whether the United States should help the government of El Salvador or stay out of the situation.

These poll results indicate that there has been at best a lukewarm public response to one of the first and most visible foreign policy initiatives of a newly elected president who promised a tougher stance on foreign policy issues than that of his predecessor.

There are also these indications of public disaffection with the Reagan policy in El Salvador:

Results from the same Gallup Poll, made public earlier this week, showed that only 2 percent of the public thinks the United States should send troops to help the government of El Salvador. Among Americans familiar with the situation, fewer than half favor sending military aid or military supplies to El Salvador, both of which the administration has done.

Mail to key congressional offices, including those of Sen. Charles H. Percy (R-Ill.), chairman of the Senate Foreign Relations Committee, and Rep. Michael Barnes (D-Md.), chairman of the House subcommittee on inter-American affairs, is running strongly against administration policy in El Salvador. "I have yet to see a letter supporting the administration on this issue," said an aide to Democratic Sen. Alan Cranston, suggesting the public sentiment in Reagan's home state of California.

At least some segments of the public are making their opposition known directly to the administration. The White House happily provides the daily or weekly mail count showing public support for the president's economic policies, but when it comes to El Salvador it directs all inquiries to the State Department, where mail on the issue is also shipped. About three weeks ago, that mail was running about 10 to 1 against the Reagan policy. Yesterday, a department spokesman said that is still the case.

These numbers suggest that, just as Secretary of State Alexander M. Haig Jr. overplayed his hand in seeking to dominate the administration's internal foreign policy-making machinery and was slapped back into place this week, the administration as a whole may have overplayed its hand in making El Salvador such a visible symbol of its determination to halt Soviet and Cuban supported insurgencies.

In particular, the increased U.S. support for the Salvadoran regime of Jose Napoleon Duarte appears to have deeply alienated large numbers of American Catholics, a voting group that gave Reagan important support last November and which remains crucial to the ongoing status. Incensed by the murder of El Salvador Archbishop Oscar Romero last March and the murders of four American Catholic women missionaries last December, the church leadership has strongly opposed increased military aid to the Duarte government and called for the correction of underlying social and economic problems in that country.

Within the White House, there has been a persistent concern about overdoing El Salvador but also apparent uncertainty on how best to handle it. That uncertainty was demonstrated by the experience of John A. Bushnell, the acting assistant secretary of state for inter-American affairs. One day Bushnell, who seemed to be speaking for the administration, complained that the press was overplaying El Salvador to the exclusion of other important foreign policy issues. A few days later, the White House disavowed his statement.

Haig has been in the middle of this and his loss in the public struggle over foreign policy turf this week may have in part reflected White House unhappiness over his handling of the El Salvador issue. It was Haig who said that El Salvador is on a communist "hit list" for takeover. It was also Haig who suggested that the three nuns and one Catholic lay worker who were murdered these last December may have been attempting to run a government roadblock.

Neither statement served the White House objective of focusing attention on the president's popular economic policies and away from the divisive subject of the proper U.S. role in El Salvador.