A bill setting the stage for interstate banking is expected to meet substantial opposition as it makes its way through Congress, both sides agree.

The proposed legislation, approved 18 to 12 Wednesday night by a House banking subcommittee, would sanction short-term regional agreements allowing commercial banks from participating states to buy banks in each others' states but barring banks from outside the region. By July 1990, banks from any state would be able to enter states participating in the compacts.

In Hill parlance, the plan is known as regional banking with an interstate "trigger." A rival proposal, which was rejected, also would have allowed regional compacts, but without a mandatory shift to full interstate banking after several years. It was nicknamed the "sunset" bill because after a three-year experiment, the legislation would have expired and Congress would have reevaluated it.

While the trigger bill, introduced by Rep. John J. La Falce (D-N.Y.), is a prologue to nationwide banking networks -- outlawed in the United States since 1927 -- the sunset bill, sponsored by Rep. Doug Barnard Jr. (D-Ga.), was an attempt to give smaller regional banks time to build themselves up without the threat of being gobbled up by money-center banks after three to five years.

On Wednesday, Barnard's sunset version was defeated 17 to 13, and committee sources said they believe it does not have the votes to pass the full committee. After that, trigger banking may run into trouble, first in the Rules Committee and then on the floor, said a Barnard aide. Rep. Claude D. Pepper (D-Fla.), chairman of the Rules Committee, has written the Banking Committee chairman, Rep. Fernand J. St Germain (D-R.I.), that he opposes the trigger bill. Barnard plans to offer the sunset version on the floor when the legislation comes before the House in early summer.

Also defeated Wednesday on a largely partisan basis was an amendment requiring banks buying others across state lines to meet local credit needs. Banks and Republicans tend to consider it antibusiness; Democrats see it as offering consumer protection. Some Democrats plan to vote against interstate banking on the floor if it does not contain consumer protection, said Rep. Mary Rose Oakar (D-Ohio).

On the Senate side, Banking Committee Chairman Jake Garn (R-Utah) opposes a trigger bill. The Senate has not yet drafted a bill.

Meanwhile, both chambers await the Supreme Court's decision on regional compacts, expected before the summer recess. If the high court upholds the right of states to keep money-center banks outside their borders, trigger legislation probably would not pass Congress, a staff aide said. But if the court upholds Citicorp and breaks down the state barriers, then the regional compacts would dissolve. At that point, the money-center banks would try to force interstate banking, and opponents would turn to Congress to stop them.