Sen. Dennis DeConcini (D-Ariz.), seeking to halt temporarily the deportation of illegal immigrants from El Salvador and Nicaragua, is planning to ask the Senate Appropriations Committee this week to approve a rider to the catch-all spending bill banning the expulsions.

By attaching the bill to the continuing resolution instead of letting it stand on its own, DeConcini said he hopes to avoid a presidential veto that is sure to come in light of President Reagan's opposition to the ban on deportations.

Although an identical version of the bill is traveling a separate track in the Senate and already has won approval in the Senate Judiciary Committee, DeConcini said he believes his rider strategy offers the best chance for passage. "I don't want to wait until next session and this is the best vehicle to do it," he said.

Wade Henderson, who heads the Washington office of the American Civil Liberties Union, which has worked for about four years on getting the bill passed, said last week he was sure of 11 votes in favor of the bill in the 29-member Senate Appropriations Committee, and was working hard at securing at least four more votes among the more moderate Republican members of the committee.

"If we can get the provision out of the {Appropriations Committee}, I think we can hold it" in the full Senate, he said.

The Washington area has the third largest concentration of illegal Central Americans in the country. They are believed to number between 80,000 and 100,000 people.

But even if DeConcini is successful in the committee, which must approve the spending bill before the end of the year, the temporary ban still faces strong opposition on the Senate floor from Sen. Alan K. Simpson (R-Wyo.).

Simpson, one of the key authors of the sweeping immigration law enacted last year, said he sees DeConcini's move as an attempt to "sneak" the measure in on the appropriations bill and plans to move to have it deleted from the spending resolution if it comes before the full Senate.

"If they want to trot that baby out, they're going to get a good debate," Simpson said last week.

He said he opposes halting the deportations of illegal Salvadorans and Nicaraguans because he considers them economic immigrants who do not warrant special immigration status. He also said he does not believe that these immigrants are in physical danger should they return home. "To believe that anyone sent back is going to be shot, killed or harassed is absurd," he said.

The House passed a similar measure during the summer that would ban the deportation of Salvadoran and Nicaraguan immigrants for about two years while the General Accounting Office studies their status in the United States and human rights conditions in their countries.

Supporters of the legislation cite two new factors that may aid their cause in the Senate: the recent escalation of political violence in El Salvador and President Jose Napoleon Duarte's strong support for the ban on deportations.

Duarte, fearing massive deportations of Salvadorans in the United States as a result of the new immigration law that provides amnesty only to illegal immigrants who have been here since Jan. 1, 1982, sent a letter to Reagan several months ago asking for an exemption for all Salvadorans.

The ban on deportations would provide relief to about 700,000 Salvadorans and Nicaraguans who are in this country illegally.

In the last few weeks, high-level Salvadoran officials as well as a myriad of U.S. religious groups, human rights organizations and some labor groups have stepped up efforts to lobby members of the Senate Appropriations Committee.

The supporters say the measure is needed because the U.S. grants very few requests for political asylum to Salvadoran immigrants, and so the immigration law effectively bars entry to the United States for most Salvadorans.

Between October 1986 and June 1987, 85 percent of asylum claims filed by Nicaraguans were granted, but only 5 percent were granted for Salvadorans, according to statistics from the Immigration and Naturalization Service.