The American voters appear ready to give President Reagan a historic reelection victory of landslide proportions Tuesday and possibly restore his working majority in the House of Representatives to go along with continued but diminished Republican control of the Senate.

As Reagan and his Democratic challenger Walter F. Mondale campaigned through the Midwest yesterday, the final Washington Post survey of political observers in all 50 states and a new Post-ABC News poll of almost 9,000 voters gives Reagan a 57-to-39 percent lead in the popular vote and the good prospect of carrying more than 45 states on Tuesday.

It points to a potential Republican loss of two or three seats from the party's 55-to-45 majority in the Senate. But it suggests that GOP candidates are poised to grab Reagan's coattails in enough House districts to restore the conservative coalition of Republicans and southern Democrats that passed the tax, budget and defense measures that were the highlight of Reagan's first year.

Sen. Jesse Helms (R-N.C.) has taken a small but clear lead over Gov. James B. Hunt Jr. (D) in the year's most expensive and headlined Senate battle. Rep. Albert Gore Jr. (D-Tenn.) is expected to gain the seat of retiring Senate Majority Leader Howard H. Baker Jr. (R-Tenn.). Rep. Tom Harkin (D-Iowa) is in front of Sen. Roger W. Jepsen (R-Iowa), while Rep. Paul Simon (D-Ill.) has at least an even chance to defeat Senate Foreign Relations Committee Chairman Charles H. Percy (R-Ill.). Upset possibilities for Republicans in Kentucky and West Virginia -- and less plausibly, Massachusetts -- could reduce the net Senate loss for the GOP.

In the largely overshadowed gubernatorial elections, Republicans are favored to pick up North Carolina, Rhode Island, Utah and West Virginia, while Democrats have a chance in close battles in North Dakota, Vermont and Washington.

As always, the House battles are hardest to read, but they are vital to Reagan's second-term legislative prospects. Unless the 99-seat Demoratic majority in the House can be cut in half by regaining the 26 seats Republicans lost in 1982, Reagan could find his mandate blunted.

With only 13 open seats to defend, Democrats have built their strategy around the strength of their individual incumbents, and still believed at week's end that they could limit their losses to 10 seats.

Martin Franks, director of the Democratic Congressional Campaign Committee, said yesterday that "if we have kept the focus on our incumbents, we'll be all right. If they the Republicans have succeeded in nationalizing the House election, we are in trouble."

Each day this past week, the evidence mounted that the concerted GOP advertising drive, urging voters to back Reagan with a Republican Congress and prevent the tax hike Mondale advocated as part of his deficit-cutting plan, was having precisely the effect Democrats feared.

Republican polls picked up a 3-point gain in the past week in the "generic" House vote for the GOP, in which voters express a preference for Congress by party, not by individual. Franks -- who was in court Friday trying without success to halt the GOP ads on grounds they violated campaign contribution ceilings to individual candidates -- conceded yesterday that "a lot of our guys say they the ads are hurting."

As planned, the coordinated offensive by the major Republican campaign committees came just as Reagan was going into high gear in his final reelection effort -- and making his presence felt in the places where he appeared. Joseph Gaylord, executive director of the National Republican Congressional Committee said, "The timing is right on this . . . . Reagan is making himself synonymous with the Republican Party."

One campaign consultant for a Democratic senatorial candidate in a state where Reagan campaigned early last week said, "If Reagan can convert voters as he did there and get them thinking about supporting his people, it's going to be a rough election night."

The tactic is a repetition of the one Republicans used in 1980 to swing 13 Senate seats to their side in a closing blitz. With most of the 14 Senate Democrats on the ballot this year appearing invulnerable, the GOP aimed its "blitz" at Democratic House members. Many of them were protected in redistricting by Democratic-controlled legislatures but, where they were not, the Republicans have gone after them.

Mondale and Geraldine A. Ferraro, the first woman vice-presidential candidate for a major party, drew big and enthusiastic crowds of their own. But private surveys by both parties showed Reagan and his running mate, Vice President Bush, have continued to expand their lead in most states during the closing week of the campaign.

Reagan has a chance at winning an unprecedented 50-state sweep, but to do so he would have to achieve the political equivalent of drawing to an inside straight. In Mondale's home state of Minnesota, which the Republican ticket has left alone, Reagan may be slightly behind. In Iowa, Oregon, Rhode Island, Hawaii and Maryland, late surveys and estimates have shown him with only the narrowest of leads.

But, of the major states on which Electoral College victories are built, Mondale is competitive only in Pennsylvania, New York, Massachusetts and Illinois, and trails in all four of them. Reagan is favored to win Georgia, Hawaii and West Virginia, which he lost to President Jimmy Carter in 1980, and could well top the 489 electoral votes he gained that year.

The Post-ABC poll, perhaps the largest preelection survey ever undertaken, completed call-back interviews between Monday and Thursday nights to 8,969 of the 11,807 voters first interviewed in the Sept. 22-Oct. 2 period. Despite the intervening events -- including two presidential debates and one vice-presidential debate -- only 10 percent of the sample said they had changed their minds about their candidate support.

That exceptionally low percentage of change indicated that most voters had made their 1984 choice early. Since the switches to and from Reagan were nearly offsetting, the current 57-to-39 percent poll result was not far from the 55-to-37 percent majority the president enjoyed in the earlier survey.

The biggest shift to Reagan came among voters in the $20,000-to-$30,000 income bracket, perhaps suggesting success for the Republican counteroffensive on the tax issue. Reagan lost ground marginally among some small voting blocs -- the supporters of 1980 independent presidential candidate John B. Anderson, for example -- in the past month, but the overriding impression of the poll is the president's across-the-board strength, spanning every region, every age group, and every income category.

He has held onto a quarter of the self-described Democrats and half the members of union households and is getting 40 percent of the Hispanic vote. He is overwhelmingly rejected by blacks and there is a gender gap of 9 percentage points, but he still leads among women.

The steadiness of the Reagan lead since last summer has long since stripped most of the drama from the presidential race and shifted much of the speculative focus to other races.

None has drawn more attention than the Hunt-Helms battle in North Carolina, which has broken all previous Senate records for spending -- and probably vituperation. Helms has taken a 49-to-46 percent lead in today's Gallup Poll for several state newspapers and private tracking polls indicate that the New Right's champion has captured what one Democrat called "the vital half-step advantage" in the closing days of the race.

But in Tennessee, the 36-year-old Gore seems certain to move to the Senate where his father once served. In Iowa, Harkin appears to have a comfortable lead over Jepsen, according to the latest polls. In Illinois, private polls on both sides show the Percy-Simon race dead even, and Republicans fear that the push for straight-ticket voting in Chicago wards and a heavy black voter turnout can sink Percy.

That could leave the Republicans with as little as a 52-to-48 majority in the Senate, but they have the opportunity to pad that a bit if their longshots come through in Kentucky or West Virginia.

In Kentucky, Jefferson County Judge Mitch McConnell (R) has made a surprising surge against Sen. Walter D. Huddleston (D) and made that seeemingly safe incumbent nervous.

In West Virginia, retiring Gov. John D. (Jay) Rockefelller IV (D) has had a similar scare thrown into him by business executive John R. Raese (R).

Earlier Republican hopes for the seat of retiring Sen. Paul E. Tsongas (D-Mass.) have faded, as controversy continues to swirl around the head of conservative businessman Raymond Shamie (R), who is opposing Lt. Gov. John F. Kerry (D) for the job.

In Texas, where the retirement of Sen. John Tower (R) opened a seat, Rep. Phil Gramm (R-Tex.), who in his earlier incarnation as a Democrat cosponsored the 1981 Reagan budget, is far enough ahead of liberal state Sen. Lloyd Doggett (D) to win -- unless the huge increase in Texas voter registration heralds an outpouring of black and Hispanic voters.

The governors' contests where party control may shift include:

* North Carolina: Rep. James G. Martin (R) narrowly leads state Attorney General Rufus Edmisten (D) in the race to succeed Hunt.

* North Dakota: Gov. Allen I. Olson (R) has been pressed hard by state Rep. George Sinner (D).

* Rhode Island: Cranston Mayor Edward DiPrete (R) is a clear favorite to defeat state Treasurer Anthony J. Solomon (D) for the seat of retiring Gov. J. Joseph Garrahy (D).

* Utah: House Speaker Norman H. Bangerter (R) leads ex-representative Wayne Owens (D) in the race to succeed retiring Gov. Scott M. Matheson (D).

* Vermont: Attorney General John J. Easton Jr. (R) and former lieutenant governor Madeleine M. Kunin (D) are in such a tight race that the votes siphoned off by three minor candidates may deny either of them the required 50 percent majority and throw the election to the Republican-controlled state House of Representatives. Gov. Richard A. Snelling (R) is retiring after four terms.

* Washington: Gov. John Spellman (R) fell far behind Pierce County Executive Booth Gardner (D) in his fight for a second term, but now seems to be coming back.

* West Virginia: Ex-governor Arch A. Moore Jr. (R) has been favored to succeed his old rival, Rockefeller, but state House Speaker Clyde M. See Jr. (D) is making it a race.

The contests for the House of Representatives are spread from Long Island Sound to the Mexican border of Arizona, with the heaviest concentration of possible turnover districts in North Carolina, Texas, Pennsylvania, New York and Illinois.

Only one committee chairman has been heavily targeted by the Republicans this year and Rep. James R. Jones (D-Okla.), the head of the House Budget Committee, appears likely to survive the Reagan landslide in Tulsa.

The three lawmakers who ran afoul of the law and their colleagues' judgment have a better chance with their constituents.

Rep. Garry E. Studds (D-Mass.), censured for a homosexual affair with a House page, is favored to win a seventh term over former Environmental Protection Agency official Lewis Crampton (R).

Rep. Daniel B. Crane (R-Ill.), censured for sexual misconduct with a female page, has an even chance of winning a fourth term over state Sen. Terry L. Bruce (D).

Rep. George Hansen (R-Idaho), who was convicted on felony counts for filing false financial disclosure documents, is trailing college professor Richard Stallings (D) in the polls, but has not been counted out by Idaho political observers.