A preliminary economic forecast by the Congressional Budget Office suggests that next year's federal deficit could exceed by nearly $30 billion the $136 billion target established by the balanced-budget law, despite last fall's deficit-cutting agreement between Congress and the Reagan administration.

The tentative projections sent to the congressional budget committees, which CBO will not make final until early next month, are less optimistic than the administration's economic projections. In preparing the president's fiscal 1989 budget request, to be submitted in mid-February, the Office of Management and Budget has used economic projections that would allow the government to reach the $136 billion deficit target set by the revised Gramm-Rudman-Hollings law.

Although the CBO economic estimate does not include a deficit forecast, House and Senate aides said the preliminary projections of economic growth in the CBO report would yield a fiscal 1989 deficit as high as $165 billion.

That would leave Congress with two unpleasant alternatives only months after it struck a deal with the White House to cut $76 billion from the anticipated deficit over two fiscal years. To reach the $136 billion target mandated for next year by the balanced-budget law, Congress would have to find spending cuts and new revenues that far exceed those called for in the accord with the White House or adopt the administration's rosier economic predictions in order to pass a budget resolution that, perhaps in theory only, reaches the deficit target.

The latter alternative seems the most likely, particularly in an election year when lawmakers can be expected to have little enthusiasm for cutting deeper in spending or to raising more taxes.

There is ample precedent for Congress to use the administration's more optimistic economic projections. Last year the House and Senate adopted the Office of Management and Budget projections in their budget resolutions. The OMB economic forecast yielded a projected deficit $25 billion smaller than would have been projected by CBO economic growth assumptions.

A consensus forecast by leading private economists yields a deficit projection of $167 billion for fiscal 1989.

In a breakfast meeting with reporters yesterday, House Majority Leader Thomas S. Foley (D-Wash.) conceded that it would be "very hard" for Congress in an election year to adopt either deeper spending cuts or higher taxes than called for in the two-year budget summit. But, said Foley, "If OMB and CBO find we are not going to meet the $136 billion target under Gramm-Rudman, we would be under great pressure to take additional action."

But under the revisions to Gramm-Rudman-Hollings adopted last year, it is the administration's budget office that will have the final word on whether Congress has missed the deficit target, a finding that would trigger automatic, across-the-board budget cuts.