"The United States is the most logical source, not only of financial support, but of political and cultural relations" for Nicaragua, a member of the Central American nation's ruling junta said here this week.

The Carter administration which has proposed a $75 million aid package for the war-damaged country, agrees with junta member Arturo Cruz. But congressional conservatives who have fought the package since last fall say that Nicaragua is already lost to the Soviet camp, and that the United States would be pouring money into a government hostile to its regional interests.

Cruz arrived here this week to press for passage of the aid. Far from Nicaragua's revolutionary image, he is a middle-aged, tie-and-suited banker, a former Inter-American Development Bank official, former head of Nicaragua's Central Bank and a self-described political moderate who "believes in the Nicaraguan revolution -- there are no 'immoderates' in that sense."

Those who charge that Nicaragua has been "infiltrated" by Soviets and Cubans, Cruz said in an interview, fall to realize that the country needs to prove it independence from long-term U.S. dominance but also desperately needs money and international support. The United States, he said, "has the opportunity to participate" and do some infiltrating of its own through requested economic and military assistance.

Since the $75 million aid package came off the State Department drawing board last August, just weeks after Nicargua's Sandinista guerrillas toppled dictator Anastasio Somoza, the proposal has been the core of a wide-ranging controversy over the future of U.S. relations with Central America.

It has consumed hundreds, perhaps thousands of hours of congressional committee testimony and floor debate, administration speeches and State Department meetings. Scores of delegations -- government officials and businessmen from Nicaragua, U.S. congressmen and interested third parties -- have traveled between the two countries gathering supporting evidence.

U.S. Ambassador to Nicaragua Lawrence Pezzullo has spent nearly as much time in Washington as he has in Managua, shepherding the aid proposal through the torturous ups and downs of the past 10 months.

The $75 million, a special supplemental aid program attached to the fiscal 1980 budget, has been under discussion so long that it now has been overtaken on the congressional agenda by another $54 million package proposed by the administration in its regular fiscal 1981 foreign aid bill.

A scheduled House floor vote on the '81 oil bill today is expected to set the stage for yet more hearings in the House Appropriations Committee on the $75 million next week.

The economic emergency after the Nicaraguan civil war that originally prompted the supplemental proposal largely has passed, although the country still is in desperate financial straits. Both the Nicaraguan and U.S. administrations now recognize that passage of the $75 million has become a psychological and political necessity for good relations, its failure likely would be a killing blow to U.S. attempts to alter traditional policy in dealing with change in the region.

That policy was marked by decades of support for political and economic elites that substituted military strength for popular backing. Despite the Carter administration's public repudiation of that policy, the United States made almost as fervent an effort to prevent a Sandinista takeover in Nicaragua as it did to push the resignation of Somoza, an historic U.S. ally.

But once the administration decided to get along with the victorious Sandinistas, it threw itself into the task with exchange of missions and public statements, supported by the supplemental aid proposal.

At that point, Congressional conservatives led by Rep. Robert Bauman (R-Md.) blamed the administration for "giving" Nicaragua to the leftist Sandinistas. They also accused the administration of failing to intervene on behalf of traditional rightist goverments and of trying to give away other similarly agitated countries such as El Salvador and Guatemala.

The conservatives have bolstered their case with references to the large Cuban presence in Nicaragua and the establishment of a Soviet Embassy, as well as a recent official Nicaraguan visit -- complete with a joint communique -- to the Soviet Union.

Nicaragua and the Carter administration note that the Sandinista government contains a number of moderates, including Cruz -- and Conservative Party member Rafael Cordova Rivas, also a new junta member, and that the U.S. aid program and stronger U.S. ties are supported by vitrually every political, business and labor group in Nicaragua.

"Nicaragua is a country that has been treated like a child for year and then grows up and wants its rights," Cruz said of Nicaragua's relations with Cuba and the Soviets.

After 10 months of the reality of governing the country, Cruz said, Nicaragua knows "we need aid, we need to be pragmatic. We are renegotiating our debt, because we want more credit. We can't ignore the international market, we would die without it."

Cruz, accompanying Foreign Minister Miguel dEscoto, saw Secretary of State Edmund Muskie on Monday.