Republican hopes of a sizable midterm election victory are falling like autumn leaves.

A state-by-state survey by The Washington Post of election prospects a month ahead of the Nov. 7 voting finds virtually no evidence of a significant Republican recovery from the defeat President Carter and the dominant Democrats administered in 1976.

The likelihood at this point is for a near standoff in the congressional contests and fewer gains for the GOP in key state races than earlier had seemed possible.

The findings-based on reports from political correspondents in each of the 50 states and interviews with national campaign officials and pollsters for both parties - come with cetain warnings. The late congressional and the tendency of most candidates to hoard their television money for the last few weeks mean that visible campaigning has barely begun iny many states. Voter apathy seems pervasive, and a low turnout could produce more surprises of the kind that have dotted the last six months of primaries.

But if the returns on Nov. 7 conform at all to the readings taken now on the races, Carter and the Democrats will enjoy an election night that, by any historical standard, will have to be accounted a sizable defensive victory.

In the governorships, where out-parties most frequently signal a national recovery, Republicans do have their best prospects for significant gains. If everything broke right, they might come close to doubling the 12 governorships they now control. But a more probable gain is in the range of six, and several of the key states that once seemed ripe for a takeover - Florida, California and Pennsylvania - now seem more likely to remain in Democratic hands.

In the Senate, where Democrats now enjoy a 68-to-32 advantage, the odds now favor a one-seat Democratic gain. Democrats are likely to replace Republicans in Nebraska, New Jersey and Oklahoma, while only in South Dakota is there equal assurance of a Republican takeover.Six other Democratic seats are in some jeapardy, but so are five other Republican seats. On balance, Majority Leader Robert C. Byrd (D-W. Va) is likely to have at least as large a majority.

In the House, where Democrats have a 287-to-146 majority with two vacanies, the Post survey identified 28 prime targets for Republican takeovers and 17 similarly inviting districts for aspiring Democrats. Not all are likely to switch, of course, and there will be upsets in districts not on this list. But the net advantage of 10 seats for the Republicans appears a reasonable guess at this point.

The estimate is in line with national opinion polls showing little, if any, diminution in the Democrat's share of the popular vote for Congress from 1976.

How well such an outcome would compare with historical averages depends on what yardstick one uses. During this century, the president's party has lost an average of four Senate seats and 34 House seasts in each mid-term election.

But, according to an analysis in Congressional Quarterly, the average loss has been much smaller in the first mid-term election following a change of party control in the White House - the situation this year. In the four elections since the New Deal that fit this defintion - 1934, 1954, 1962 and 1970 - there was barely a dent in the status quo. The president's party lost only six House seats and gained four Senate seats on the average.

If this pattern provides a rationale for prospective Republican dissapointments, it does not obliviate the fact that the hoped-for GOP comeback does not seem to be materializing. Questions for Carter

Republicans have raised sums for an off-year election and deployed what Democrats concede is the best organization field effort they have ever seen the opposition mount. With inflation the dominant concern of voters and tax-cutting moves springing up like weeds following the success of Proposition 13 in California, the issues seemed to be cutting in the Republican direction. Until Camp David, Carter seemed a weak president, being shunned by many in his own party, and there was a Korean bribery scandal to cloud the reputation of Democratic Congress.

With all that, if Republicans emerge with the skimpy gains that now seem indicated, there are bound to be new concerns expressed about the future of the party.

But not everything in the Post findings, summarized on Pages C4 and C5, falls in the category of good news for Jimmy Carter.

The most openly ambitious of his intraparty rivals, California Gov. Edmund G (Jerry) Brown, Jr. has moved well out fron at the moment in his up-and-down race for reelection. If Brown defeats Atty. Gen. Evelle Younger by the margin the polls now measure, his eagerness to challenge Carter in the 1960 primaries would doubtless be increased.

Two prospective Republican challengers who must pass tests at the polls this year appear to be winning. Illinois Gov. James R. Thompson and Tennessee Sen. Howard H. Baker Jr. have likely enough opposition to make it difficult for them to approach their most recent victory margins. But neither is likely to be derailed.

Nor can Carter automatically take comfort from every Democratic victory. Texas Atty. Gen. John Hill is favored to succeed Gov. Dolph Briscoe, an early Carter admirer. But Hill last week said the president's farm and energy policies would make it hard for him to support Carter for renomination, and he is likely to be much more aggresive than Briscoe in advertising his difference with the White House. But the larger story in the reports coming in on the mid-term election concerns the difficulties the Republicans are encountering in trying to regain the ground they lost in Congress and the states in the Watergate year of 1974, plus the loss of the White House in 1976.

Given the indenpendence of most members of Congress in their voting on legislative issues, party ratios no longer have the importance they once did. But it would be ominous for the Republican to have no senator form traditionally Republican Nebraska. It would be costly to lose the single senate seats Republicans now hold in New Jersey, Massachusetts, Michigan, Texas and Virginia. Not all of these setbacks will occur, but some are most inevitable.

Even more threatening is the continuing Republican weakness at the state and local level indicated in these reports. Republican National Chairman Bill Brock has pointed his party toward rebuilding from the ground up, and has specifically focused on gubernational and legislative races this year.

These battlegrounds are doubly important because of the massive reapportionment and redistricting that will follow unless Republicans recover strongly in the state house elections this year, they could be wiped out by redistricting for the next decade.

His aides are no longer making specific claims about the number of legislatures in which they hope to overturn Democratic majorities. But the prospects in the gubernatorial races look considerably dimmer than they did just a few moths ago.

California, Florida, Pennsylvania and Texas - all of which have been targeted for major Republican efforts - now seem likely to have Democratic governors again. That is doubly disappointing, for, in each case, there were factors at work that made political turnovers seem likely. But Brown has moved agilely to become the "Proposition 13" governor in California, and in the other three states new Democratic faces have emerged who seem capable of withstanding even the well-financed drives energetic Republican challenges are mounting.

Further, Ohio's veteran Gov. James A. Rhodes (R) is now trailing in his race, and Michigan Gov. William G. Milliken (R) is, as usual, expecting a close contest.

New York could give the Republicans their biggest gubernatorial victory. Assembly minority leader Perry Dureya (R) is leading Gov. Hugh L. Carey (D), but the Dureya campaign causes his supporters as much nervousness as it does Carey.

Republicans have gubernatorial takeover possibilities in such middle-sized states as Massachusetts, Wisconsin, Minesota and Tennesse. But most of the numerical gains they expect in the state house races will come in places where the political stakes are minimal.

As in the past, the elephant's diet appears likely to be small potatoes.