Virginia State Sen. Wiley F. Mitchell Jr. (R-Alexandria) gets a lot of mail, but none is so plaintive as that which crosses his desk now that the General Assembly is about to grapple with the results of Ronald Reagan's budget cuts.

Advocacy groups for the mentally retarded, the physically handicapped, the aged, autistic children, family services and numerous others have inundated legislative offices like his with newsletters and telephone calls, all sounding the alarm over cuts in federal spending and hoping they can persuade the state to make up some of the loss.

"But I don't see how we can stretch three feet of cloth to cover a four-foot gap," said Mitchell, whose conservative state, unlike Maryland and the District, anguishes less in public over the financial squeeze on the poor and disadvantaged.

President Reagan has cut more than $40 billion from the federal budget since his inauguration, all in an effort to strengthen the economy and hold down a budget he wants to balance. While programs for the poor, unemployed and other disadvantaged persons have been among the hardest hit, losses in federal funding also are jeopardizing state and local government ability to provide services in these areas.

In Virginia, with both capital and general funds stretched, even the most sympathetic of officials say the state will be hard pressed to make up any shortfalls from the more than $250 million in Reagan budget cuts to the state over the next two years. Outgoing Gov. John N. Dalton will present his final two-year budget to the new session of the General Assembly today.

"I expect to bleed quite a lot myself along with some of these groups that are going to be affected," said Joseph L. Fisher, the former congressman from Arlington who Democrat Gov.-elect Charles S. Robb has named secretary of human resources.

Spokesmen for Dalton have said they expect 75 percent of the federal cuts to hit the state's "people programs," but advocacy groups for the poor and disadvantaged are hoping that Robb will somehow restore the social service programs Dalton aims to phase out.

Robb has kept quiet about his own budget plans since his election, however, prefering to wait for Dalton's budget. Aides to Robb, who promised during the campaign to strenuously resist tax increases, say the new governor is not likely to try to make up the lost federal monies.

"The words the governor Robb used to me were that he didn't expect there would have to be many changes" in the Dalton budget, said Wayne F. Anderson, Robb's new secretary of administration and finance.

Fisher, who talks of "cutting right to the bone," doesn't rule out asking the legislature for more money if his department is left with critical needs unmet. But the state's legislators have been saying for weeks that raising taxes is just not the Virginia way.

"We'll spend exactly the revenues we take in, and there will be no revenues raised by outside sources, no taxes raised to make up for the deficits," said State Sen. Richard L. Saslaw (D-Fairfax). "Essentially, it's going to be the survival of the fittest. That's the way it is here."

The probable lone exception to that maxim will be the new tax monies legislators are expected to seek for transportation needs. State Sen. Adelard L. Brault (D-Fairfax), the senior member of Northern Virginia's legislative delegation, argues that funds will be needed for highway construction and maintenance and for Metro.

Reagan's budget reductions will cost Virginia about $95 million during this fiscal year. The Dalton administration estimates that Reagan cuts and various state revenue losses will be between $400 to $600 million over the next 2 1/2 years. Much of these losses result from Reagan cuts and state tax losses that the cuts will cause and expected state revenue losses because of the recession. Dalton has projected the loss of $207 million in state revenues over the next three years due only to Reagan administraton cuts in corporate, individual and estate taxes.

The state's approximately $6 billion annual budget has already been squeezed by the loss of more than $70 million in federal revenue sharing monies in the past two years. The state also has experienced a significant decline in tax revenues, has frozen spending for state construction programs and has laid off employes. Health care, income assistance and other programs have been severely curtailed.

Virginia's budget problems have received different reactions from government leaders, depending on their political party. Democrats predict the worst but take comfort in blaming a Republican administration for economic hardships. Republicans have tried to put on the best face, arguing that the cuts will help the economy.

"We'll make do with what we have," said John F. Herrity, Republican chairman of Fairfax County Board of Supervisors. Virginia legislators say they have what Brault calls an "eyes-wide-open" attitude toward the budget crunch.

The Democratic-controlled assembly seldom tinkers with any governor's budget, but there are indications that its budget committees may flex their muscles more this session. The Democratic leadership balked last year at giving Dalton more authority to shift budget funds around from department to department. Robb, the first elected Demoractic governor in 16 years, takes office Saturday and is expected to have a more cordial relationship with the legislature than Dalton has had recently.

But a reapportionment dispute will force House members to run for reelection next fall. Virginia politicians have never done that during a budget year, and campaign considerations will likely make them hesitant to make unpopular budget decisions, legislators acknowledge.

"The delegates are used to having a year's reprieve after a budget session," said Brault, wryly adding that House legislators won't feel at all like voting for any tax increases or cuts in services, and then going home to answer for their actions at the polls.