President Reagan launched a new drive yesterday for a $100 million aid package to rebels fighting the Sandinista government of Nicaragua, including a resumption of military aid. But congressional Democrats immediately expressed opposition and White House officials acknowledged that the proposal would be Reagan's most difficult legislative battle this year.

In a meeting with Republican congressional leaders, Reagan said "we have to do more" to help the Nicaraguan rebels, also known as contras. "You can't fight attack helicopters piloted by Cubans with Band-Aids and mosquito nets," he added. The administration is seeking $30 million in overt, nonlethal aid, such as clothing and medical supplies, and $70 million in covert military assistance for the contras, officials said.

Congress barred military aid in 1984 but last year approved $27 million in nonlethal assistance, which expires March 31.

Senate Foreign Relations Committee Chairman Richard G. Lugar (R-Ind.) said Reagan also wants to lift all restrictions placed by Congress on the aid last year when it ordered the administration to channel the funds through the State Department, instead of the Defense Department or the Central Intelligence Agency.

A spokesman said Lugar has scheduled a hearing next week on the $30 million request for nonlethal aid, and may begin mark-up on legislation the following week.

The $70 million request for covert military aid falls under the jurisdiction of the Senate Select Committee on Intelligence where Chairman David F. Durenberger (R-Minn.) has expressed opposition to aid to the rebels. Durenberger declined to comment yesterday. In the House, Rep. Lee Hamilton (D-Ind.), chairman of the Permanent Select Committee on Intelligence, has in the past opposed covert aid and also declined comment yesterday.

White House officials said some details about the Reagan proposal are still being worked out, including whether the $70 million can be "reprogrammed" from existing funds.

Defense Secretary Caspar W. Weinberger told congressional leaders yesterday that military programs have been "cut to the bone" and more money cannot be squeezed out for the contras, but Secretary of State George P. Shultz said some key lawmakers have urged the administration not to seek additional funds for the military aid, but rather to redirect money already appropriated.

White House officials said Reagan intends to use his trip to Grenada Thursday, as well as meetings with lawmakers this week, to press the issue. He is expected to make a formal presentation of the aid package next week or the week after, officials said.

Lugar said Reagan was offering a "good program," but other Republicans, as well as Reagan's advisers, predicted that the package would be the president's toughest legislative fight in 1986.

Rep. William S. Broomfield (Mich.), ranking Republican on the House Foreign Affairs Committee, said, "I think it's going to be a very hard program to sell. The opposition is already gearing up to oppose military aid and conditions in Congress haven't changed that much" since the last vote.

Broomfield said the administration's request is likely to be cut. "Congress is not going to turn its back on the contras," he said. "But obviously their chances are better of getting humanitarian aid," rather than military aid.

Rep. Michael D. Barnes (D-Md.), chairman of the House Foreign Affairs subcommittee on Western Hemisphere affairs, said he was "very disappointed" in Reagan's proposal. "It's a direct slap in the face to our friends in the hemisphere who are concerned about the course we are taking," he said. Barnes noted that the House has five times rejected military aid to the rebels, but stopped short of predicting that Reagan's proposal would fail.

A spokesman for the Nicaraguan Embassy here said the request was "very detrimental" to the Contadora regional peace process.