Unless last-minute lightning strikes, the controversial Endangered Species Act and the federal office that administers it will go out of business at the end of tomorrow.

And when it does, Interior's procedures for considering special protection for hundreds of endangered plants and animals will stop. The list now includes about 700 endangered species, and about 2,000 more are being considered.

The law caught in a congressional tangle that has prevented final action on proposals to extend it for three years beyond tomorrow's expiration.

Although congressional action appears imminent, the arm of the Department of Interior that oversees the law will be without money and authority and most of its endangered-species work will stop.

"I'm sure Congress has no intention of leaving us dangling. They may change our style, but we're confident we'll get an authorization or appropriation soon," said Keith M. Schreiner, associate director of Interior's Fish and Wildlife Service.

Schreiner said tomorrow's cutoff would affect about 200 full-and part-time Interior employes who administer the law to protect imperiled flora and fauna. They will work on other tasks while Congress makes up its mind about extending the act.

As Schreiner noted, the major question is one of style - whether, and how much, protection should be given to species that critics say are of little value to mankind.

Underlying that, however, is a more volatile issue. Many members of Congress fear that the act will be used increasingly to stop federal public works projects they consider important to their districts.

Their fears were stimulated by a Supreme Court decision earlier this year, which held that the Tennessee Valley Authority had proceded illegally with its $116 million Tellico Dam project by not considering the endangered snail darter, a tiny and nearly extinct fish.

Of thousands of federal projects considered under the act, Tellico is the only one to have been stopped because builders failed to accommodate an endangered species.

The Tellico decision came in June as Congress was considering extension of the 1973 act. Since that time, with pressure mounting to weaken it, extension legislation has been caught in the Capitol Hill tangle.

The Senate passed a three-year extension, but bowed to the critics by including a new review process that could result in overruling Interior in the most complicated conflicts between projects and species.

The House Merchant Marine and Fisheries Committee, after months of haggling over details, this month passed a different and more complex extension bill. It, too, would allow protections of the act to be abandoned in cases of "irresolvable conflicts."

An effort to clear the bill for House action failed this week when the Rules Committee failed to come up with a quorum. Another effort to get a rule will be made next week.

An aide to Rep. John D Dingell (D-Mich.), a key supporter of the bill, said that the House leadership has assured its backing on the move to get a final vote.

Throughout the legislative haggling, Dingell and other conservation-minded members have been fighting a holding action - offering compromises but attempting to avert moves to gut or kill the act.

Most environmental groups, unhappy with the compromises but aware of the heavy pressures for change, are supporting the watered-down extension.

Even with that, the bill faces more fire when it reaches the House floor, Rep. Robin Beard (R-Tenn.) is poised with a package of amendments that Dingell says would be a disaster for the legislation.