Financially strapped Nicaragua is expected to win International Monetary Fund approval Monday for $65 million in special loans despite last minute congressional pressure on the Carter administration to block the action.

The United States used its large IMFvoting bloc to kill a similar loan to Nicaragua last November. Informed sources said, although the administration remains at odds with President Anastasio Somoza, it is unlikely to oppose his government's current request at an IMF board meeting Monday.

The Somoza government hopes to use the money not only to replenish its near-bankrupt treasury, but also to encourage foreign commerical lenders to reschedule payments on approximately $65 million it owes them this year and another $23 million left unpaid from 1978.

Nicaragua's financial troubles stem from a year and a half of continuing turmoil in which most sectors of the country-ranging from leftist guerrillas to conservative business, political and church groups-have tried to oust the 43-year-old Somoza government.

In addition to a civil war last September and continued sporadic fighting between Sandinista guerrrillas and the government's National Guard, the government's economic difficulties have been increased by nationwide strikes, lack of investment and substantial capital flight.

At least 42 U.S. congressmen signed letters yesterday to Carter, Secretary of State Cyrus Vance and Treasury Secretary Michael Blumenthal, whose office is directly in charge of U.S. votes at the IMF, saying the loans would "impede the resolution of [Nicaragua's] political and economic crisis."

One of the three Nicaragua loans under consideration Monday, called a standby credit agreement, is conditional on an IMF-imposed fiscal austerity program that the letters said will increase unemployment there and cut back on "already limited government spending on social services."

Additionally, the letters said, the loans would "constitute an international vote of confidence for the discredited Somoza regime." The letters originated in the office of Rep. Tom Harkin (D-Iowa), who has led a House campaign against U.S. aid to Somoza.

In a statement yesterday, Sen. Edward M. Kennedy (D-Mass.) also urged U.S. opposition to the IMF loans, and called on the administration to take other sanctions against the Somoza government.

Administration sources said, however, that the United States is unlikely to oppose the IMF actions because that would open it to charges of "politicizing" the ostensibly apolitical lending institution.

Last year, Nicaragua applied for two IMF loans. The first application, in August for a standby agreement, was withdrawn when fighting broke out there in September. The second, for a relatively routine "compensatory financing" loan designed to help the balance of payments, was postponed and effectively killed by a U.S.-led vote in November.

Somoza and his supporters in the U.S. Congress accused the administration of manipulating the IMF. The United States at that time was involved in a mediation effort between Somoza and his opponents and had privately asked Somoza to resign.

Others in the State Department and among IMF member nations also voiced concern at the U.S. tactic.

This time, Nicaragua has applied for three loans totaling $65 million. According to an informed administration source, little U.S. opposition is likely unless extreme technical grounds can be found for voting no.

"The outrage would be even greater" this time, the source said. "It's not seen as a Nicaragua issue," he said, but rather a question of "mucking around with an important body like the IMF.