Arturo Cruz, leader of the democratic opposition to Nicaragua's Sandinista government, yesterday shifted position to endorse continued U.S. funding for the armed opposition "contra" rebels.

Cruz said it would be "a terrible political mistake" to end the U.S. aid program before the Soviet bloc halts aid to the Nicaraguan government.

He also asked other governments and private organizations to demand steps toward democratic reform as a condition for further aid to Nicaragua.

The announcement was a boost for the Reagan administration, which regards continued contra attacks as crucial to its policy of pressuring the leftist Sandinistas toward regional peace talks and domestic political concessions.

The statement represents a considerable shift for Cruz, a former Central Bank president and former member of the Sandinista government. He previously said the contra program had provided the Sandinistas with an excuse to tighten domestic repression.

"Yes, it is used politically by the Sandinistas," Cruz said at a news conference. "But you have to go back to the cost-effect" analysis . . . this is a civil war." If U.S. backing stops, Cruz said, "the Sandinistas would not get rid of the Soviets or become more democratic; it would be just the opposite."

However, he rejected an economic boycott or other stiff U.S. sanctions because "the effect would be terrible" for civilians and for the Nicaraguan economy.

With Cruz was Pedro Joaquin Chamorro, editor of Nicaragua's lone opposition newspaper, La Prensa, who last month went into voluntary exile in Costa Rica to protest growing censorship and travel restrictions. He ridiculed "revolutionary tourists" from the United States and elsewhere who follow Sandinista guides through Nicaragua and believe what they are told, he said.

"One of the wonders of the revolution is that there is still a newspaper publishing," he said. "They don't see that 50 percent of what we want to publish is censored."

Chamorro charged that outbound Nicaraguans are required to pay twice for their tickets, once in dollars and once in Nicaraguan currency. "The price now is too high for most Nicaraguans," Chamorro said.

Cruz said he still wants an end to the contra program but only in the context of an overall settlement involving freedom for the press, political parties and unions; a political amnesty for dissidents and "a more rational" foreign policy.

That position is fundamentally the same as the contras'. He denied any "organic relationship" to them but said there is "a commonality of purpose." A spokesman for the largest rebel group, Bosco Matamoros of the Nicaraguan Democratic Force (FDN), said Cruz's remarks had "clarified" the situation and were "useful for the cause of democracy, which is our cause too."

Ambassador Carlos Tunnermann of Nicaragua said Cruz "has publicly revealed that he is now acting in a coordinated manner with the armed counterrevolution." He said the Sandinistas "always suspected that he [Cruz] was being used, possibly without realizing it," by the contras and added that "commonality of purpose" meant that Cruz had adopted the contras' avowed purpose of overthrowing the Sandinista government.

The Reagan administration hopes to persuade Congress to provide another $14 million next month for the contras, who have used more than $75 million in U.S. aid to grow from 500 ragtag rebels in 1981 to an estimated 14,000 insurgents. But Congress ended the aid last May, and the issue promises to be among the most controversial of the new session. Cruz said he would testify to Congress on the need for the program if asked to do so.

The FDN yesterday issued a report alleging hundreds of cases of displacement, torture, arbitrary arrest and murder of civilians by the Sandinista government.

The 11-page list was in part a response to a report last week from the Council on Hemispheric Affairs (COHA), a private educational and human rights organization. COHA charged the contras with routinely torturing and murdering hundreds of civilians suspected of backing the Sandinista government.