Prince George's County School Superintendent John Murphy dropped a little bombshell last week in his continuing hard-sell campaign for increased school funds when he announced his endorsement of a controversial new energy tax.

Legislators, county officials and others were caught off-guard by the endorsement, which came as Murphy announced his proposed $381 million school budget for the fiscal year that ends in June 1987.

The energy tax idea has not been a popular one. County Executive Parris Glendening was practically run out of town when he forwarded it as a last-minute proposal to the Prince George's delegation during the last General Assembly session.

Business people warn that the new surcharge would be a cost that would be passed on to the consumer. And Prince George's County tax limitation activists, who believe that they lost enough ground when a six-year-old property tax cap was lifted in 1984, argue that the energy tax would be another unnecessary burden.

Murphy, calling the prospects for new county revenues "dim," issued his call for the energy tax as a challenge to officials who will be forced in an election year to decide whether breaking the cardinal campaign rule against supporting new taxes can be offset by public sentiment favoring improved schools.

Glendening, who said he did not ask Murphy to make the energy tax pitch, remains noncommittal about whether he will venture forth again with the plan that earned him such legislative enmity last year.

But Prince George's legislators who were interviewed after Murphy's announcement last week did not rule out the possibility that they might look a little more kindly on a proposal designed to fill the gap between the approximately $12 million they requested from Gov. Harry Hughes for magnet schools and the $3.5 million Hughes included in the $8.2 billion state budget he released this week.

"The energy tax is not unthinkable," said Prince George's Del. Timothy F. Maloney, one of those who called Glendening's efforts on behalf of the tax last year a "last-minute charade." Maloney, a Democrat, said Hughes' proposal "is not the last you're going to see this session on education funding."

Also less adamantly opposed to the notion of a new tax was Democratic Del. David Bird, a guiding force behind the efforts that spawned TRIM (Tax Reform Initiative by Marylanders), the 1978 charter amendment that set a county tax revenue ceiling.

Bird said he would "be inclined" to support an energy tax proposal if it is placed only on commercial establishments. "If it's an energy tax on everybody, that's substantially different."

Such softening of the opposition gives the measure's traditional backers cautious hope, but the measure's supporters do not want the holders of the state purse strings to feel that the county is not doing all that it can to help itself. It is hard for Prince George's to cry poor, these backers say, when it rejects fiscal alternatives like the energy tax.

"We had agreed this would be plan B," said Del. Joan Pitkin, who along with fellow Democratic Dels. Albert Russell Wynn, Frank B. Pesci Sr. and William C. Bevan was drafting an energy tax authorization bill that has yet to be introduced. "The bill is on the back burner because we wanted to see what the governor was going to come up with."

However, Sen. Thomas V. Mike Miller Jr., a Democrat and chairman of the county's Senate delegation, does not believe that the energy tax proposal will survive.

He has joined the fight for magnet school funds and said the county could reap millions more from its share of the restructured education aid formula Hughes has included in his budget.

"John Murphy is going to have to be satisfied with that for 1986," said. "There is always going to be a gap there," he added, noting that the county will have to absorb the loss of nearly $12 million in revenue sharing money that was deleted from the federal budget.

Glendening said that the county could stand to gain between $3 million and $4 million if the governor's beefed-up education aid redistri- bution plan gains legislative approval.

Murphy's budget proposal, which represents a 6.5 percent increase over this year's $357.9 million budget, includes $12.9 million for the magnet school desegregation program and $2.5 million to reduce class size with additional teachers.

Glendening said he has already directed his finance staff to draw up a budget that would include an increase of more than 12 percent for schools even though the revenue sources are not yet clear.

"I can't see going one more year where we just inch by . . . just making do," he said.

Any revenue-producing plan, including the energy tax proposal, will eventually have to go to the County Council for its approval. Some legislators believe that even if they vote to authorize a new tax, the council will balk.

"It probably was not the thing to do for the superintendent to begin recommending a particular tax," said council Chairman William B. Amonett. "I wouldn't do just anything to find the money . We have an election year to go through.

"The General Assembly is aware of that," he added. "I don't think they'll give us something that would be unreasonable to get passed."