The Soviet transport Agostinho Neto, carrying an estimated 5,000 tons of war supplies, arrived unannounced in the Nicaraguan port of Corinto last week -- increasing the probability that the Democratic-controlled Congress will continue military funding for the contras.

The ship carried peanuts compared with the $580 million in war materiel the Soviet Union and its satellites gave the Marxist Sandinista regime in Managua last year. But it makes prophetic a classified U.S. intelligence estimate that total Soviet-bloc war goods expected to be shipped this year will set a new record, along with a roughly equal amount of nonlethal goods.

Combining that estimate with realities of presidential politics, the Democratic Party is not about to use its control of Congress to terminate U.S. aid (now running at perhaps 15 percent of the Soviet rate). On the contrary, political reality dictates that Congress may well break new ground and vote aid for 18 months, bridging the first six months of the new administration.

The man-bites-dog prospect of the Democratic Congress suddenly giving an embattled President Reagan's contra policy a break contradicts conventional wisdom inside the Washington Beltway. But its hard foundation of political reality is grounded in a transformed political atmosphere.

A major element in that transformation was the retirement of Speaker Thomas P. O'Neill, removing from Democratic power a highly emotional contra foe whose approach was implacable stonewalling. His hatred of the contras was so consuming that he refused even to hold routine meetings of Democratic leaders on the subject.

O'Neill's successor, Speaker Jim Wright, is not saying much about Nicaragua beyond wanting ''diplomatic'' approaches. However, close friends insist that secretly he may favor aid because of genuine concern about communist gains in Central America and the inevitable Soviet influence that comes with it -- major concerns in his own state of Texas.

But what should worry Wright more is another factor in the transformed atmosphere: political danger to his party if another aid cutoff snuffs out the contras. This time, in the wake of the Iran-contra affair, there would be no back-room money deals with foreign governments and rich American conservatives to sustain the anti-Sandinista guerrilla army.

Former Virginia governor Charles S. Robb, chairman of the mainstream Democratic Leadership Council, poignantly stated the case for continued military aid in a recent Manhattan talk to the Foreign Policy Association: ''Turning on aid one year and off the next is tantamount to playing with their {the contras'} lives.''

He wants a multi-year aid package to guarantee ''constant pressure'' on the Sandinistas. If Congress will not make a ''clear and continuing'' commitment to aid, ''it should terminate the program altogether -- and be prepared to accept the consequences.''

Polls show no increase in voter opposition to the contras despite the unending, acrimonious Iran-contra hearings.

Each Soviet vessel that arrives with more MI 25 ''flying tank'' helicopters and other lethal arms to fight off the U.S.-backed contras raises the political question: Can the United States sit back and watch? The administration has not yet decided either how much to ask when the current $100 million funding runs out at the end of September or when the right moment will come to send the request to Congress.

But pressures are increasing for a request of at least $125 million a year for either 18 months or two years. That's in line with what one high official calls ''contra reality,'' which gets a little stronger each day as new Soviet arms pour into Nicaragua.