The Democratic budget nearing final approval in Congress may miss the fiscal 1988 deficit target by $40 billion, $14 billion more than originally thought, according to a preliminary re-estimate being prepared by the Congressional Budget Office.

Though the new forecast will remain tentative until the CBO meets with its panel of economic advisers next week, congressional sources said that CBO officials are estimating that the Democratic budget would yield a deficit in the range of $148 billion in the fiscal year that begins Oct. 1. That is well above the current estimate of $134 billion and would leave Congress further from its deficit goal of $108 billion set for the year by the 1985 Gramm-Rudman-Hollings law.

Though the new preliminary estimate, if it holds, would make Congress' attempts at deficit reduction more difficult, it could facilitate a budget compromise with the Reagan administration.

The new estimate is based on CBO projections of an increase in the base-line deficit -- the amount of red ink that would occur if Congress adheres to current-year spending policies -- and is largely attributed to expected higher interest rates that raise the cost of federal borrowing.

CBO had previously estimated the base-line deficit at $171 billion, a figure that the House and Senate Budget committees used in preparing their budgets now being negotiated in a conference committee. Both versions of the fiscal 1988 budget anticipate reducing the base-line deficit by about $36 billion, to what legislators had believed would be near $134 billion, through a combination of spending cuts and increased taxes.

Neither the House nor Senate budget comes close to meeting the $108 billion deficit target set for fiscal 1988 by the Gramm-Rudman-Hollings deficit-reduction law.

The CBO normally re-estimates in August the budget adopted by Congress, but is doing work earlier this year because of Congress' efforts to devise a package of budget-process changes that could include revision of the Gramm-Rudman-Hollings deficit targets.

One Senate budget aide said yesterday that if the CBO preliminary forecast holds, it would "complicate measurably" efforts to meet the deficit target. But the aide said it might bring needed pressure on all sides, including the White House, to come together on a grand compromise involving fiscal 1988 spending and budget-process revision.

The administration and Congress are in the early stages of negotiating budget revisions that may be tied to legislation needed to increase the federal debt limit in mid-July.

The administration, which has vowed to veto the roughly $18 billion tax increase in the Democratic budgets, has insisted that it will not negotiate on the 1988 budget until Congress agrees to budget revisions. The revision package would force Congress to adhere to its budget blueprint when it enacts spending bills.

Furthermore, all sides in the budget process disagree on what should constitute budget reform, including how to replace the automatic spending-cut mechanism struck from Gramm-Rudman-Hollings by the Supreme Court.

House Democrats are pushing for a mechanism that would put political pressure on the president to negotiate spending and revenue decisions with Congress or face deep cuts in defense spending. In addition, they want to change the yearly Gramm-Rudman-Hollings deficit targets so that Congress would be required only to reduce the deficit by a flat $36 billion per year.

That Democratic position could be bolstered by the prospect of the congressional budget missing the fiscal 1988 target by $40 billion, even after Congress has voted for $36 billion in deficit reductions.