WASHINGTON--Congressional opponents of military aid to Nicaragua's Contra rebels say they have virtually no chance of blocking the release of $40 million in arms aid approved last year.

In the first fight of the year over the Reagan administration's Contra policy, the House is to vote Wednesday on a Democratic bid to delay release of the $40 million final installment for six months while the administration provides a full accounting of missing Contra funds, private as well as public.

The proposal is designed to spotlight alleged corruption in the ranks of the Contras, who are battling Nicaragua's leftist Sandinista government.

But Republicans denounced it as a backdoor way to bottle up aid promised when Congress approved $100 million in Contra assistance last year.

The issue may come before the Senate as early as Thursday, but in different form.

There the vote likely will be on an up-or-down disapproval resolution. While Democrats control the Senate and may be able to pass the measure by a simple majority, vote counters admit they cannot rally the two-thirds majority needed to override a certain Reagan veto.

Therefore, they say will not make a major effort, saving their energy for battles later in the year over the additional $105 million Reagan has requested for the Contras in fiscal 1988 and for legislation sponsored by Sen. Christopher Dodd (D-Conn.) to end Contra aid permanently.

"There is no way we can override the president's veto of a disapproval resolution," Senate Majority Leader Robert C. Byrd, D-W. Va., said last week. "The $105 million down the road is another question."

Dodd said that as far as he can see, the $40 million installment for the Contras is "a done deal" and Reagan is "going to get the money."

"There certainly are not enough votes to override a presidential veto," Dodd said. "I wouldn't make the stand on that issue."

Many lawmakers say that blocking the $40 million would break a promise Congress made last year when it voted to provide a full $100 million.

No promises were made on aid beyond that and Dodd and others said there likely will be many more votes against further assistance, especially in light of the Iran-Contra turmoil involving the sale of arms to Iran and the purported diversion of profits to the Contras.