President Reagan today denounced the government of Nicaragua and appealed for financial aid for the anti-Sandinista "freedom fighters" whom he called "our brothers."

"These brave men and women deserve our help," Reagan said in his weekly radio speech. "They do not ask for troops but only for our technical and financial support and supplies. We cannot turn from them in their moment of need. To do so would be to betray our centuries-old dedication to supporting those who struggle for freedom."

Underscoring the importance Reagan places on aiding the Nicaraguan "contras," a senior administration official briefed reporters after the speech. The official said the speech launched an effort by the president to win congressional approval of $14 million in assistance to the rebels.

Under a congressional agreement reached in October, the administration is prohibited from aiding the contras financially. But $14 million would be available from the fiscal 1985 budget if both houses agreed to lift the ban.

The official said Congress had "dug in its heels" against covert aid and acknowledged that it would be an uphill fight for the funds. He said Reagan had reviewed the situation and decided to lead an "educational" campaign to rally public support for Congress to approve the aid.

"The only problem that exists now is the Soviet client state of Nicaragua in Central America," the official said. "What we're trying to do is prevent a final consolidation of the Sandinistas as a Marxist-Leninist society."

However, a leading congressional opponent of covert aid to the contras said today in Washington that the president's campaign "is unlikely to sway the Congress."

"Congress has voted four times in opposition to this program, and there is no evidence that that will change," said Rep. Michael D. Barnes (D-Md.), chairman of the House Foreign Affairs subcommittee with jurisdiction over Central America. "He is very persuasive, but public opinion is very strong in opposition to this program."

Reagan's decision to become involved in the battle for contra aid appeared to be a sign that he does not intend to allow the upcoming U.S.-Soviet arms talks or generally improved relations with the Soviets to muffle his anticommunism or his commitment to the anti-Sandinista rebels.

In an unusually emotional speech, the president said that "one of the most inspiring developments of recent years is the move against communism and toward freedom that is sweeping the world." He paid tribute to Soviet dissidents, the Solidarity independent trade union movement in Poland and to "freedom fighters" in Afghanistan, Ethiopia, Cambodia and Angola.

"These brave men and women are fighting to undo the infamous Brezhnev Doctrine, which says that once a nation falls into the darkness of communist tyranny it can never again see the light of freedom," Reagan said.

He was referring to the "limited sovereignty" doctrine of the late Soviet leader Leonid I. Brezhnev that the Soviets invoked in crushing Czechoslovakian liberalization in 1968.

Reagan called the Sandinista government "a Marxist-Leninist clique that broke the hearts of the freedom-loving people of their country by imposing a brutal dictatorship soon after taking control in 1979."

"Functioning as a satellite of the Soviet Union and Cuba, they moved quickly to suppress internal dissent, clamp down on a free press, persecute the church and labor unions and betray their pledge to hold free elections," Reagan said.

"Now they're exporting drugs to poison our youth and linking up with the terrorists of Iran, Libya, the Red Brigades and the PLO. The Sandinistas aren't democrats but communists . . . , creators of a fortress Nicaragua that intends to export communism beyond its borders.'

The president asserted that aiding the contras was both legal and "totally consistent with our history." He compared the efforts of the rebels with those of colonial troops during the American revolution who were aided by French forces.