Washington suburban legislators today extracted a promise from Gov. Harry R. Hughes to give their counties another $3.5 million to help cover their portion of Metro's operating losses this year.

Hughes' latest offer, according to an administration aide, was intended to appease Washington area lawmakers who have threatened to tie up the final days of this session by pressing for a sales tax rebate measure strongly opposed by the governor.

The large delegations from Prince George's and Montgomery counties also have worried Hughes by announcing plans to "get even" with Baltimore legislators who helped defeat Metro financing bills in a Senate committee Wednesday night.

"The domino effect of Rpince George's and Montgomery county delegations could be devastating for many programs," said Senate Majority Leader Rosalie S. Abrams, who warned Hughes of the implications and encouraged him to up the ante for Metro.

The additional $3.5 million promised by Jughes today would commit the state to more than half of the $29.5 million share of Metro deficits owed by Prince George's and Montgomery. With federal aid, the counties, would only have to pay $4 million of the debt.

Legislators from the two Washington suburbs have successfully whittled down their obligation since the start of this session by stepping up their demands on the administration. Hughes first offered them $6.5 million and the increased his offer twice.

"We made it clear we were serious," said Sen. Victor L. Crawford (D-Montgomery). "We feel we have leverage with at least 13 of the 15 votes from our two counties. Harry is not looking to have his first session tied up by a knock-down, drag-out fight."

Hughes took another step to prevent such a fight by promising Crawford and his allies that he would reconsider his opposition to legislation that would authorize the state to pay $156 million for the second phase of Metro construction.

That bill was killed in a Senate committee two day ago, but could easily be revived with Hughes' support, said Sen. Laurence Levitan (D-Montgomery), whose role as committee chairman significantly increases the clout of the Washington suburbs.

Some commitment from the state to contribute to Metro's future construction costs has been the top priority of Washington area legislators this session because of the nearly $300 million budgetary surplus projected for the end of this fiscal year.

The state already has appropriated $160 million for the first phase of Metro, but the payments are scheduled to run out next year and Prince George's and Montgomery want some assurance that more money will become available for their 1980 budgets.

Until today, Washington suburban lawmakers have pressed for full state support of Metro's second phase and a second measure that would commit the state to a continuing revenue source to pick up the rail system's annual operating losses.

If Hughes agrees to support financing of Metro's future construction, however, the legislators from Prince George's and Montgomery say they will drop their pursuit of their second objective -- a measure to assure state help for Metro's yearly operating losses.

That measure -- vehemently opposed by Hughes -- would return to the state's 24 subdivisions a fraction of the state sales taxes to be used for transportation purposes. The bill was killed in committee, but sponsors say they could resurrect it.

It was the prospect of reviving that sales tax rebate measure, according to a Hughes aide, that prompted the governor to "sweeten up the pot" for Washington area legislators with the latest $3.5 million offer for operating costs.

Abrams reported to Hughes that the two Washington area counties were picking up support from other parts of the state -- especially from Baltimore County which stands to gain a large rebate if the measure passes.

"Harry was mainly trying to stop that coalition from forming," said the governor's aide, noting that Hughes does not want to commit an important source of state revenues for transportation purposes. "He was afraid it (the sales tax measure) was going to snowball."