Sen. Alan K. Simpson (R-Wyo.) has agreed not to block attempts early next year to bring up for consideration on the Senate floor a bill that would ban the deportation of illegal Salvadoran and Nicaraguan immigrants.

The unusual written agreement with Sen. Dennis DeConcini (D-Ariz.) removes a major hurdle to getting the bill, which has cleared the House and the Senate Judiciary Committee, onto the Senate floor for debate.

Before a bill can come up for discussion, senators must approve a motion to consider.

Simpson and DeConcini also have agreed to meet with religious, civil rights and immigrant advocacy groups to discuss the bill before it goes to the Senate floor, which could be as early as February, DeConcini said yesterday.

Simpson has been the most forceful opponent of the proposed legislation that would halt the deportation of Salvadorans and Nicaraguan immigrants for two years while the General Accounting Office studies their status in the United States and human rights conditions in their native countries.

"I see this as removing the last significant obstacle to a fair consideration of the bill," said Wade Henderson, who heads the Washington office of the American Civil Liberties Union and has been working for four years to have the legislation enacted.

In return, DeConcini agreed not to try to have the bill included as a rider on the catchall spending bill.

DeConcini had feared that Simpson would filibuster a preliminary motion to bring the bill up for consideration, which could have kept it in limbo.

He said that while Simpson still may filibuster an attempt to get the bill passed and other senators could try to keep it from even being considered, the bill now stands a much better chance of being brought up for discussion. "That might serve the purpose of persuading some undecided members or get the administration to listen to the debate up here and see what the issues are," DeConcini said.

President Reagan and some administration officials have opposed the measure, although top Salvadoran officials have requested that illegal Salvadorans in the United States not be returned home because of the economic problems that could ensue.

There are about 500,000 Salvadorans in the United States, and the Washington area is said to have the third-largest concentration in the country.

Many of these immigrants arrived here after 1982 and are ineligible for amnesty under the immigration law.