President Reagan, lobbying for $100 million in aid to the rebels fighting the Sandinista government of Nicaragua, said yesterday he had received fresh evidence that Sandinista security agents have dressed as counterrevolutionary "freedom fighters" to "murder and mutilate" Nicaraguan civilians.

Reagan used his regular Saturday radio address for the second straight week to build a case for contra aid. He spoke from Camp David, where he also was putting the finishing touches on tonight's nationally televised address aimed at stirring up enough public support to persuade the House this week to approve $70 million in military aid and $30 million in nonlethal assistance for the contras.

United Press International said a poll of House and Senate members indicated the president's request faces an uphill fight in both chambers unless he will compromise.

White House officials said yesterday that Reagan does not intend to mention any such compromise in tonight's address but might change his stance if he loses the crucial vote in the House this week.

"To move as far as we have on an issue as contentious as this is rather remarkable," said Dennis Thomas, deputy to White House chief of staff Donald T. Regan, asserting that the president is picking up votes.

White House and congressional vote counters agreed that Reagan is about 15 votes short in the Democratic-controlled House. Reagan is expected to engage in intensive personal lobbying of House members from Tuesday until the vote.

Reagan said in his radio address that he had sent special U.S. envoys on 49 trips to Central America, but "have yet to see any concrete results. The time has come to give the freedom fighters what they need for their struggle . . . . "

In an apparent response to charges by human rights groups that the contras have mutilated and killed civilians, Reagan said he had received "new intelligence" that "Tomas Borge, the communist interior minister, is engaging in a brutal campaign to bring the freedom fighters into discredit. You see, Borge's communist operatives dress in freedom fighter uniforms, go into the countryside and murder and mutiliate ordinary Nicaraguans."

Rep. Thomas S. Foley (D-Wash.), delivering the Democratic response to the radio address, said: "Incredibly, we may be walking into the quicksand with our eyes wide open" by fueling the war in Nicaragua rather than pressing harder for a negotiated settlement. "The president of the United States is the one person who can breathe new life into the process," Foley said.

Foley said that if the United States provided military aid to the contras, the next steps could well be U.S. advisers in the field and then "American combat forces themselves" in Nicaragua because the U.S. intelligence community agrees that "only U.S. forces could truly resolve the conflict in Nicaragua on a military basis."