President Reagan asked Congress yesterday for $100 million in aid to rebels fighting the leftist Sandinista government of Nicaragua, launching what is expected to be another bitter battle over U.S. policy in Central America.

Critics and backers of the rebels have started mobilizing nationwide campaigns to promote their views of the program.

It would give the counterrevolutionaries $30 million in nonlethal aid -- such as food, clothing, medicine and trucks -- and give President Reagan control of another $70 million to use "for any kind of assistance he deems appropriate," according to an official fact sheet.

It is an open secret that those funds would be used for military aid to the estimated 20,000 rebels, although Reagan did not say so.

"Approval . . . will permit me to use any department or agency in the executive branch, including agencies involved in intelligence activities, in carrying out programs and activities to assist the Nicaraguan democratic resistance," he told Congress.

"Our experience with the Sandinistas over 6 1/2 years points unmistakably to the need to accompany diplomatic policy with substantial pressure" for change, he said.

However, Reagan pledged that he would not regard approval as authorizing use of U.S. armed forces against the Nicaraguan government.

The funds would be transferred from already allocated Defense Department funds in six installments over the next 18 months, through the end of fiscal 1987. Reagan said he would move later to address the "impairment in defense readiness" caused by the transfer.

Initial congressional reaction was wary but interested. Reagan removed a possible source of opposition by asking for a transfer of funds rather than for new money under strictures of the Gramm-Rudman-Hollings balanced-budget law.

In addition, by asking for spending authority rather than direct military aid, he also defused at least temporarily the question of covert versus overt assistance, which has raised the issues of Central Intelligence Agency involvement and whether to withdraw recognition of Nicaragua.

A senior administration official told reporters at a White House briefing that "the only way to get the Sandinistas to the bargaining table is to convince them they do not have the option of a military solution" of defeating the contras. "You aren't going to do it with sweet talk," he said.

If Congress rejects the aid, it would leave the administration only two options: "introduction of U.S. forces . . . or just walking away," the official said.

"They got our message about not asking for new money," said Rep. Dave McCurdy (D-Okla.), a member of the House Permanent Select Committee on Intelligence and instrumental in forging last year's package of $27 million in nonlethal aid to the contras, which expires March 31.

He was also one of 31 swing voters on that package who asked Reagan earlier this month to delay submitting his new proposal until the Central American nations hold a March summit meeting.

That group met yesterday to discuss the new proposal. "As the package is presented we are not going to accept it," McCurdy said. "We are working on other options."

David F. Durenberger (R-Minn.), chairman of the Senate Select Committee on Intelligence, said he would seek to "clarify the request" by attaching a prohibition against using the $70 million for lethal purposes.

He said he thinks that he has the votes to do that.

Rep. David R. Obey (D-Wis.), chairman of the House Appropriations subcommittee on foreign operations, said he opposes the request.

"The issue is whether you will turn the president into a king," he said. "I am disinclined."