President Reagan tentatively plans to ask Congress for a $140 million package of aid to El Salvador divided more or less evenly between military and economic assistance in an attempt to overcome congressional resistance to an exclusively military increase, reliable sources said yesterday.

The sources said that, as of last night, the administration was working on the assumption that Congress will not go along with the $110 million proposal for emergency military assistance floated by Defense Secretary Caspar W. Weinberger on Tuesday and that a much greater share of any fund request will have to be earmarked for economic aid.

A clear signal that congressional opposition remains strong came yesterday from House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.), who told reporters:

"I really don't think the votes are there for $110 million unless they show us circumstances much worse than we think now."

Reagan is expected to reveal the exact size and nature of his request today in a speech before the National Association of Manufacturers. The sources said he was still consulting with his top advisers and Republican congressional leaders last night.

According to the sources, the administration, after encountering a largely negative congressional reaction to the idea a supplemental appropriation of $110 million in military aid, had weighed the possibility of adding $30 million in economic assistance.

However, the sources continued, that idea was dropped, on the assumption that $30 million would be insufficient to overcome congressional opposition.

Administration policy makers then reportedly decided to stick with the $140 million figure, but to reserve a substantial amount--perhaps as much as 35 or 40 percent--for economic aid.

If Congress buys that idea, the sources noted, it would give Reagan more than the $60 million in military aid that he originally indicated was necessary for the Salvadoran government to continue its campaign against guerrilla foes. But it would be less than the $110 million put forward by Weinberger on Tuesday.

The sources said that the administration's likely fallback position will be to try for the lesser amount of military funds in its supplemental request and then seek to make up all or part of the difference by reprogramming military assistance funds already appropriated and earmarked for other countries.

The administration had been reluctant to follow that course because it means taking money away from other programs that it considers vital. However, the sources said, extensive consultations over the past week force it to conclude that there is no hope of winning the requisite support on Capitol Hill unless the president puts more emphasis on economic aid, ensures that no U.S. military advisers are sent into combat situations and takes steps to explore the possibilities of a political and diplomatic solution.

In a related development, the head of the National Conference of Catholic Bishops made public a letter to Reagan expressing concern about recent statements by Secretary of State George P. Shultz and Vice President Bush that have been interpreted as critical of the church's role in Central America.

In the letter, Archbishop John R. Roach of St. Paul and Minneapolis rejected "any hint" that the church's attitudes are based on "an alien ideology" like Marxism, and he requested a meeting between church and administration representatives to dispel "confusion" and "honest misunderstandings" about the matter.