Key anti-Sandinista political leaders joined the armed Nicaraguan rebel movement today in a new demand for dialogue with the Sandinista leadership on ways to bring peace to Nicaragua.

The declaration, indirectly broadening political support among exiles for the guerrilla war there, coincided with a vigorous campaign by the Reagan administration to persuade Congress to renew CIA funding for the rebels. In that context, it seemed partly designed to strengthen administration arguments that the guerrillas represent the spearhead of broad-based political opposition to the Sandinista government.

The declaration unveiled here after weeks of negotiations, entitled "Document of the Nicaraguan Resistance on National Dialogue," was intended as the expression of a political accord among exile leaders both within and outside the guerrilla war that has been raging in Nicaragua for three years, its drafters said.

For the first time, it brought former presidential candidate Arturo Cruz into a grouping with the main guerrilla organization fighting in Nicaragua, the Nicaraguan Democratic Force. Cruz, a former Sandinista official, previously had avoided open association with the force and inisted today that his signature on the document did not make him part of any political front designed to facilitate renewed U.S. funding for the guerrillas.

"We are not lobbying," he said. "This has nothing to do with the aid from Congress."

Two leaders of rebel forces not allied to the Nicaraguan Democratic Force, Eden Pastora and Brooklyn Rivera, declined to sign the document.

Leaders of the Nicaraguan Democratic Force appeared pleased that Cruz had joined them in the declaration of political goals and what they called a "last chance" challenge to the Sandinistas to open a national dialogue on finding peaceful ways to settle the conflict. Cruz, who has lived many years in Washington, is regarded by guerrilla leaders as a forceful ally in dealing with U.S. congressional leaders.

Adolfo Calero, the force's chief political figure, pointed out that Cruz's association with the political document marked a departure and added: "If the Sandinistas do not reply, I would interpret this to mean Mr. Cruz agrees with the Nicaraguan Democratic Force in reinforcing our quest for military opposition."

Another leader of the force, Indalecio Rodriguez, said that even though the document is only political, it could form the basis for an umbrella group that could receive U.S. funds if Congress refuses to give money directly to the guerrilla forces.

The document, which repeated political goals long held by Cruz and the other signers, gave the Sandinista leadership until March 20 to agree to the idea of a dialogue under the aegis of the Roman Catholic Church's bishops' conference.

"If on April 20, 1985, the national dialogue has not begun or has not advanced in a substantial and visible way, it will be definitively suspended by the National Resistance, and with that the possibilities of a peaceful settlement to the national crisis will be considered ended," it added.

This seemed to be a threat to the Sandinistas that if they continue to refuse the demand for dialogue, Cruz and the other political leaders who have avoided joining the guerrilla movement will lend their full political weight to the war.

The Sandinistas previously have rejected similar demands for national dialogue on opposition political goals such as dissolving the link between the Army and state and the ruling Sandinista National Liberation Front. Other goals include pluralist elections, a mixed economy and expulsion of foreign military advisers.

At the same time, rebel leaders pointed out that President Daniel Ortega recently has made a series of gestures designed to renew the Contadora peace negotiations, indicating that the Sandinistas are seeking some opening. The way to settle the conflict, the document said, is not negotiations between the Sandinistas and other governments, but between the Sandinistas and their Nicaraguan opponents, armed as well as unarmed.

Signing the document, along with Cruz and Calero, were rebel leaders Alfonso Robelo and Fernando Chamorro and Pedro Joaquin Chamorro, former editor of the opposition newspaper La Prensa, who left Nicaragua for exile late last year.