AT MID-CAMPAIGN, the 1980 presidential election is still Ronald Reagan's to lose. But with 30 days to go, it has also -- and emphatically -- become a contest which is Jimmy Carter's to win.

The Washington Post's state-by-state survey indicates that, as of this week, Reagan has wrapped up or is leading in 28 states with 283 electoral votes -- 13 more than he needs for victory. Carter has moved out front in 14 states and the District of Columbia with 151 votes. Eight states with 104 votes are in the tossup category.

The survey -- based on reports from political correspondents in the 50 states, private and public polls and interviews by Post reporters with national and state officials in both parties -- clearly demonstrates that independent contender John B. Anderson is being squeezed out of contention.

There is now no state where the evidence indicates Anderson has a realistic prospect of winning any electoral votes. But there are many where his votes could spell the difference between Reagan and Carter -- usually, but not always, to the president's disadvantage.

In New England, moderate Republicans' defections from Reagan to Anderson are opening opportunities for Carter to win states he lost in 1976. He is leading now in Maine and is even or almost even in Connecticut and Vermont, all of which went to Jerry Ford four years ago.

On the other hand, the reluctance of dissident Democrats to forego their protest vote for Anderson is blocking Carter in such states as Pennsylvania, New Jersey, Michigan, Oregon, Washington and Wisconsin, in all of which Reagan is currently out front. Even in the South, where Anderson is weakest, he has enough support to make the difference in such closely contested states as Kentucky, Tennessee and Louisiana.

The turn into October saw the two biggest electoral prizes -- California and New York -- moving off the center of the playing field, the former seemingly secure for Reagan and the latter trending unmistakably to Carter.

But the rivals' polling margins in such key states as Texas, Illinois, Florida, Michigan, Pennsylvania and New Jersey are so narrow that in a real sense the election is still out there, waiting to be won.

Reagan's strategic advantage for the closing weeks is most visible by comparing today's alignment of states with the actual results in 1976, when a shift of 28 electoral votes would have made Ford the winner and Carter the loser.

As of today, Reagan is rated even with or ahead of Carter in 10 states with 150 electoral votes which Ford lost: Delaware, Florida, Kentucky, Louisiana, Missouri, Ohio, Pennsylvania, Tennessee, Texas and Wisconsin.

Carter is rated even with or ahead of Reagan in only three states Ford won last time: Connecticut, Illinois and Maine. They have 38 votes.

That erosion of Carter's base results in part from the Anderson slough-off, but even more from Reagan's success in playing on the disillusionment of former Carter supporters -- many of them blue-collar families -- with the president's economic policies and performance in office.

Reagan established a bedrock base of 142 electoral votes in his own Western region and the normally Republican states of the Midwest while Carter was still struggling to close off the challenge from Sen. Edward M. Kennedy (D-Mass.). From convention time on, the president has known he had to win two-thirds of the electoral votes Reagan did not have nailed down, while the challenger could be more selective in his campaign focus.

But the Post survey demonstrates that Carter went a long way toward solving that strategic problem during September. His margins in the 15 states where he now leads -- and the trends within those states -- make his 151 voters look secure.

Thus, while Reagan's current support is almost twice as broad as Carter's, it is -- almost literally -- only half as deep.And that is both significant and potentially ominous for a man running against an incumbent president.

Pollsters in both campaigns agree that the decision-making process by which voters finally reach their ballot-box judgment is far less advanced this year than in most presidential elections. Several pollsters have found the percentage of undecided voters in their states increasing in the last two weeks -- the reverse of the normal pattern.

In part, they speculate, that is because the presence of Anderson in the race has delayed and confused the ultimate choice. In part, it is because Reagan and Carter have yet to meet in face-to-face debate.In part, it is because voters see serious liabilities in both men. And in part, it is because each man was occupied for much of September correcting his own errors, rather than defining the choices in the way he wanted the voters to see them.

But it would be a mistake to think that September was wasted in the Carter campaign. The reports from the states suggest a very important pattern. Financially, organizationally and psychologically, Reagan began with considerable advantages. The Republicans were much more "up" for the election than the Democrats.

But from state after state, the evidence is that during September three things were happening: There was a steady movement of Kennedy backers, encouraged by the senator's own words and acts, to support of Carter. There was a gradual mobilization of the rusty machinery that Democrats and their union, teacher, black and brown allies use to turn out their votes. And there was a gradual, occasionally precipitous, decline in Anderson's support -- to the point that even in Massachusetts, where 24 percent in a poll supported Anderson, only 2 percent said they thought he would win.

Taken together, these trends have moved Carter into a position where he can try to win the election -- not just wait for Reagan to lose it.

Fortuitously for Carter, the issues seem to be moving in his direction. Public optimism about the economic future is increasing. Despite the rise in interest rates, Friday's economic news of a decline in both unemployment and inflation rates was exactly what the Carter camp prayed for. Simultaneously, the continuing focus on tensions in the Persian Gulf raises the level of interest in foreign policy -- where Carter makes his best showing against Reagan.

Knowing the advantage any president possesses for exploiting the machinery of government, creating and timing events at home or abroad and shaping their coverage, the current optimism in the Reagan camp is understandably tempered by a fear that Carter may find a way to seize the initiative in the campaign's final weeks.

Outside his immediate circle, key Republicans argue strongly that Reagan must build a bigger cushion of voter support against such likely developments; that he must seek ways to expand his constituency, not just hold the votes he has; that he must take chances, rather than strive to minimize the risks of political errors.

The concern is heightened by the realization that the undecided voters are likely to break Carter's way -- those, that is, who do not conclude the choice is not worth bestirring themselves. They are disproportionately low-income individuals, living in areas where most of their neighbors are Democrats. But the odds are great that many of the undecided will not vote. Enthusiasm is still lacking in the Carter campaign; Republicans have more volunteers, as well as more dollars, and in almost every state, Reagan's margin improves when polls segregate the likely voters from the larger pool of registered voters.

While Reagan and Carter brood about such matters, scores of other politicians are running their own races -- and most of them, in this era of ticket-splitting, are proceeding on a separate track from the presidential contest.

There is no evidence of a dramatic upsurge in Republican strength or a massive turnover in Congress or the state capitols. But there are some lively contests.

Of the 13 states electing governors, only North Dakota and Missouri seem halfway likely to switch parties. Neither of the incumbent Democrats in those states, however, is worse than even money for another term. Democrats are likely to remain in control of eight states, from Rhode Island to Utah, and Republicans to remain in command in three others. Two millionaires who would like to be president some day, Govs. John D. (Jay) Rockefeller IV (D) of West Virginia and Pierre S. (Pete) DuPont (R) of Delaware, are favored to gain reelection this year and keep their hopes alive.

The Senate contests have drawn more attention, in part because organized conservative groups, some aligned with business political action committees and others with "New Right" fundamentalist churches and antiabortion organizations, are attempting to defeat several liberal senators up this year.

One of them, George McGovern of South Dakota, is trailing, and three others -- Frank Church of Idaho, Birch Bayh of Indiana and John C. Culver of Iowa -- are in very close, tough races.

But the Senate pattern is more complex than the New Right vs. Old Liberal script suggests. The dean of Senate Democrats, Warren G. Magnuson of Washington, is in big trouble, in part because he has spent half his 75 years in Congress, and in part because his opponent, state Attorney General Slade Gorton, is a moderate Republican with broad voter appeal. Except for the age issue, the same can be said of John A. Durkin in New Hampshire, who looked reasonably safe until a credible moderate Republican, former state Attorney General Warren Rudman, slipped through several conservatives in the GOP primary to challenge him.

Ironically, one conservative "triumph" -- the defeat of Sen. Jacob K. Javits of New York by conservative Alphonse D'Amato in the Republican primary -- may well lead to the election of Rep. Elizabeth Holtzman (D), who, on her record, could easily be the most liberal member of the Senate.

Overall, the consensus estimate is that Republicans may gain two to four seats in a chamber where Democrats now outnumber them, 58-41. Other than Holtzman, the best Democratic prospects for gains are in vancant seats in Oklahoma and Pennsylvania. The top Republican targets are in Alaska, Florida, Idaho, Indiana, Iowa, New Hampshire, South Dakota and Washington.

The House contests are, as usual, their own booming confusion. There is a race in Louisiana where two Democrats named Buddy are the only contestants, and a race in New Mexico where the courts have let the Democrats replace an incumbent who died, but refused to allow the Republicans to file anyone against the replacement.

There are several races where the opponents will spend $1 million between them and a few where candidates are refusing PAC money -- even at the risk of being drastically outspent.

The Republicans currently guess that they could gain as many as 35 seats, which would be a big boost in a body where they are outnumbered 274 to 159, with two vacancies. But they would probably settle right now for a net of 25, and Democrats say that is about the maximum loss they can see for their side.

What is unusual this year is that Republicans are running strong campaigns against some of the senior and most influential Democrats in the House -- and will probably beat some of them.

The endangered targets include House Majority Whip John Brademas of Indiana. Ways and Means Committee Chairman Al Ullman of Oregon, Congressional Campaign Committee Chairman Jim Corman of California, Interior Committee Chairman Morris K. Udall of Arizona and House Administration Committee Chairman Frank Thompson Jr. of New Jersey.

Thompson is also one of the six House members involved in the Abscam investigation. Like Reps. John M. Murphy of New York and Raymond F. Lederer of Pennsylvania, Thompson is awaiting trial on charges he has vigorously denied. All three are slight favorites in November.

Rep. Richard Kelly (R) of Florida was not so lucky. He has yet to stand trial, but he lost in the Sept. 9 primary. Rep. John Jenrette Jr. (D) of South Carolina is on trial currently -- and home-state observers are not sure even acquittal would save his seat.

Rep. Michael (Ozzie) Myers (D) of Pennsylvania was convicted, ousted from the House and is a distinct underdog in his bid for reelection in Philadelphia.

The tolerance for error voters are being asked to extend to candidates for every office from the presidency on down is being sorely tested in Myers' case -- even in the City of Brotherly Love.