The morning-after shock has set in on Capitol Hill as a reconvening Congress tries to assess the full menace of the monster it has created with the Gramm-Rudman-Hollings automatic budget-slashing machine.

The most apt description of the legislation comes from Boston College Prof. Barry Bluestone. What we're really talking about, he said, is "Gramm-Rambo."

Rep. Les Aspin of Wisconsin, Democratic chairman of the House Armed Services Committee, agrees, dubbing G-R-H "First Blood, Part I." Aspin says that G-R-H in two years will wipe out the military buildup in progress since 1983.

Congress, the White House and the public have been introduced to the realities of the G-R-H mess by the $11.7 billion in civilian -- and defense -- program cuts that will take effect March 1 for fiscal 1986. To continue to meet G-R-H targets calling for an actual budget balance by fiscal 1991, Robert Eisner of Northwestern University said here the other day, could cause "the worst economic disaster we have had in many years."

The irrationality of automatic across-the- board cuts (and these are "only" 4 to 5 percent) was brought home by the realization that they will cost Uncle Sam revenue by clipping back the Internal Revenue Service's ability to collect taxes. They will leave some American embassies abroad vulnerable to terrorist attack because construction efforts to protect them will be abandoned or delayed. Until the comptroller general stepped in, G-R-H was about to cause a default on the bonds financing Washington's subway system. The list of stupidities is too long to quote fully.

But this month's G-R-H cuts provide just a foretaste of what lies ahead Oct. 1 to meet the G-R-H deficit target of $144 billion fr fiscal 1987.

Here is the devastating arithmetic, as compiled by senior staff people on Capitol Hill. From the current deficit estimate of $220 billion for fiscal 1986, substract the mandated $11.7 billion reduction. That would reduce this year's deficit (everything else staying equal) to $208 billion. Thus, to reach the G-R-H target of $144 billion for fiscal 1987, there will have to be additional cuts of $64 billion -- five to six times the reductions the agencies are now scrounging to meet.

But wait a moment, says a Joint Economic Committee economist: What happens if the economy proves to be weaker this year than the 4 percent economic growth rate projected by the Reagan administration, as much of the business, financial and academic community thinks it will be?

On Aug. 15, according to the G-R-H timetable, a budget "snapshot" re-estimating economic assumptions, will be forwarded to the General Accounting Office by the Congressional Budget Office, and the administration's Office of Management and Budget.

"Suppose the growth rate is pro percent," says a leading Hill economist. "Since each one point of GNP is worth $18 billion in tax receipts, that would add $27 billion to the budget reductions needed to meet the Gramm- Rudman deficit target of $144 billion for fiscal 1987."

That would mean a budget cut not of $64 billion on Oct. 1, but $91 billion. In the real world, such a budget amputation could not take place.

Even if the required budget cut under Gramm- Rudman on Oct. 1 is no worse than the $64 billion now foreseen, most observers believe that the president and leading Capitol Hill Republicans would either suspend G-R-H, as Sen. Pete Domenici (R-N.M.) suggests, or agree to a tax increase that would allow tolerable reductions in both defense and nondefense programs.

Four prominent economists -- Eisner, former Nixon Economic Council Chairman Paul McCracken, Alan Blinder of Princeton and John Makin American Enterprise Institute -- last week proposed a goal of cutting the $200 billion deficit by about half, or to around $100 billion, in the next five years.

But even that more reasonable approach would require President Reagan to give way on his so-far-adamant stand on tax increases. There are hints that this could happen. One can be read into the note of desperation sounded by Domenici in a Washington Post article. He warns that unless al scenario means that Republicans and Democrats up for reelection this year who voted for G-R-H will fall like flies in the wake of a government paralysis and a stock-market crash. Another clue was a little-noticed statement by McCracken, currently a member of President Reagan's outside economic advisory board.

"My guess is that we have to have additional revenues," McCracken said flatly, "and that will be recognized before the end of the congressional session."

In reality, that is the only way out of the G-R- H mess, short of jeopardizing the nation's military security and aborting the proper civilian role of government. And the vehicle for a tax increase is readily at hand: the so-called tax-reform bill. Having already passed the House, it is easily changed into a revenue-raising bill.