Despite efforts to delay it, the Federal Home Loan Bank Board yesterday formally proposed that savings and loan associations in the Washington metropolitan area be permitted to establish branch offices across state lines.

At the same time, in order to assuage critics, the board announced a lengthening of the regulation issuing process that effectively postpones any further action until after the release of a federal study on branching and appointment of new board members.

The proponents of branching, namely the District-based thrift institutions, have argued it would provide a convenience for suburban customers as well as a source of funds for rehabilitating the inner city. The opponents, mainly small suburban S&Ls, have countered that interstate branching in greater Washington would result in unfair competition and fewer urban loans.

Due to the sensitivity of the issue, not only locally but nationwide where it is seen as a precedent, the bank board took the unusual step of issuing a preliminary proposal last February. Yesterday's action gave interested parties until Oct. 15 to commet. The unusually long comment period - coming on the heels of a three month comment period in which about 9,000 replies were received - was requested late Wednesday by Sens. William Proxmire (D.-Wis.), Paul Sarbanes (D.-Md.) and Jake Garn (R.-Utah).

The legislators asked the board to defer action until after the President delivers his tudy on the McFadden Act (prohibiting interstate branching) to Congress in September. They also requested a delay until a successor to FHLBB Chairman Robert McKinney is selected. McKinney told reporters yesterday he intends to leave by the end of this month and will play "a national role relative to certain areas of our economy" in Jimmy Carter's relection campaign.

"It would be most unfortuate if, on an issue of this magnitude, the new board felt itself bound in the slightest by a rule proposed by the existing board," the senators wrote McKinney after a hearing on the nomination of Andrew DePrete, a Rhode Island lawyer, to a seat on the board. According to Sarbanes, DePrete gave an ambiguous answer when asked his position on interstate branching in this area. "His answer didn't represent a fully considered opinion because he hasn't been on the board," said the senator.

The Proposed reguation would allow branching, by any means, including merger, by any federal S&L with either a home or branch office in Meropolitan Washington, so long as the branches also in that area. No S&L could shift its home office from one jurisdiction to another. Though no limit would be set on the number of branches, only three applications for new branches should be made in one year.

McKinney stressed repeatedly that the proposal for interstate branching stems from the District's unique position as a highly urbanized city without suburban areas in the same jurisdiction. However, he did admit it might be seen as a test case by other commercial and savings banks seeking branches.

Indeed, Charles Daniel, president of Union First National Bank, said yesterday that the action would force the District's bankers to make proposals to Congress and regulatory agencies that would allow them the same branching authority.