Transportation Secretary Drew Lewis threw cold water yesterday on key features of a congressional plan to rescue Washington's crumbling Union Sation, telling lawmakers that, while the idea is good, the necessary money cannot be squeezed out of his department's austere budget.

The plan, proposed earlier this year, calls for turning most of the monumental 76-year-old structure into a complex of privately financed and developed shops and offices. The government's role would include completing the station's unfinished parking garages and ramps and improving the small Amtrak rail terminal, possibly by extending the tracks back to the old concourse area.

Lewis, testifying on a bill that would shift responsibility for Union Station from the Interior Department to the Transportation Department, said that a government partnership with private developers will "probably make this into a worthwhile commercial project." He later called the concept "exciting."

But, in his testimoney before the Senate Environment and Public Works Committee, Lewis said he opposes a proposal by the D.C. Department of Transporation to divert $35 million in federal funds from the city's interstate highway construction program to complete the parking ramp.

"We would like to see [the money] used on the interstate," he said.

The Reagan administration already has slashed $23 million earmarked by the Carter administration for Union Station improvements as part of the Northeast Corridor rail modernization program. Lewis said that existing funds set aside for the Washington-to-Boston improvement project "cannot absorb the costs of developing Union Station."

Lewis endorsed the station's transfer to his department and supported the approach proposed in a seperate bill that would authorize seperate new funding.

Lewis also urged that engineering and marketing studies, which he said would cost up to $1 million and take about six months, be made for the project.

The fate of Union Station, uncertain for years, has been in serious doubt since last February when heavy rains poured through fisures in its roof, causing plaster to fall from the ceiling and leading officials of Interior's National Park Service to close it to the public.

Since then, railroad passengers have been forced to walk as far as a quarter of a mile around the building to reach the small Amtrak station located in the track area.

Yesterday's testimony did not deal with the cost of roof repairs, which are being done by the park service with available appropriations.

Both Thomas M. Downs, D.C. transporation director, and Richard R. Hite, deputy assistant Interior secretary for policy, criticized Lewis' rejection of the use of interstate highway moeny to complete the parking garage. Downs said the project is regarded as a traffic interceptor for the nearby Center Leg Freeway (I-395).

Lawrence D. Gibson, vice president of Amtrak, supporting the rescue plan, said the existing station handles 4 million passengers a year -- one-third more than Baltimore-Washington International Airport -- and will be woefully cramped with an expected double of rail travelers by 1990.

In prepared testimony, Gilson said the proposed diversion of interstate highway funds "will be disastorous if it does not have the full and enthusiastic backing" of all affected parties.