The Reagan electoral juggernaut roared through the Midwest yesterday, just as the polls had predicted, winning 127 of the region's 137 electoral votes and sputtering only in Minnesota, the home state of Walter F. Mondale.

It was a stunning victory for President Reagan in an area whose farm and factory economies suffered more than any other region during the 1981-1982 recession. But recent signs of recovery and the president's magnetic coattails buoyed GOP hopes of retaining their Senate margin (now 55 to 45) and picking up new House seats.

In at least one case -- Iowa -- it did not work. Reagan carried the Hawkeye State's eight electoral votes, but his last-minute campaign efforts were not enough to save freshman Sen. Roger W. Jepsen (R) from defeat at the hands of Rep. Tom Harkin (D).

And in neighboring Illinois, there were early signs that the Reagan magic was not all-inclusive. The president cruised to easy victory there, but a fellow Republican, Sen. Charles H. Percy, was trailing Rep. Paul Simon (D), and the Senate race was too close to call early this morning.

But another Republican, Sen. Rudy Boschwitz of Minnesota, once considered vulnerable, easily defeated Secretary of State Joan Anderson Growe (D), a veteran politician with a habit of upsetting Republicans.

Republicans also had been expected to keep governorships in Indiana, North Dakota and Missouri. The case was closed early in Indiana, where Robert D. Orr easily clinched a second term. John Ashcroft (R) won in Missouri, but in North Dakota, incumbent Allen I. Olson trailed George Sinner (D).

Elsewhere in the region, as expected, Senate incumbents Nancy Landon Kassebaum (R) in Kansas, Larry Pressler (R) in South Dakota and J. James Exon (D) in Nebraska were comfortable winners. Sen. Carl Levin (D) was ahead in Michigan.

But Republicans, counting on an assist from Reagan's popularity, were salivating over the prospect of reducing the Democrats' edge in the 114 House seats up for grabs. Democrats held 60 seats going into yesterday's voting, with seven seats open and Republicans holding the rest.

Of the 13 midwestern House races in which an incumbent was considered "troubled," 11 seats were held by Democrats -- opening the possibility of significant GOP gains if Reagan's coattails were long enough.

A battle royal, for example, was going on in southwestern Iowa, where Republican Jim Ross Lightfoot, a former farm radio broadcaster, was regarded as a slight favorite to take the seat being vacated by Harkin.

In Illinois, Democratic Rep. Lane Evans was getting a tough GOP challenge, but fellow freshman Dick Durbin (D) won reelection. Kenneth J. Gray, coming out of retirement, was neck-and-neck in the race to keep the Simon seat in Democratic hands.

Illinois Republican Daniel B. Crane, hurt by his House censure for sexual peccadillos, was reported a loser to state Sen. Terry L. Bruce (D).

House Democrats Philip R. Sharp and freshman Frank McCloskey were in uphill fights to retain seats in Indiana, and a GOP incumbent, John Hiler, was locked in a close run against Michael P. Barnes (D) in the South Bend area.

Missouri Democrat Harold L. Volkmer won a tight race, while fellow Democrat Robert A. Young, from the St. Louis suburbs, was in a close contest. In Kansas, Republican Jan Meyers won the seat of retiring Rep. Larry Winn Jr. (R).

Republicans also targeted Michigan seats held by Reps. Donald J. Albosta and Bob Carr. Albosta lost and Carr was leading early this morning. Illinois

Reagan handily won Illinois' 24 electoral votes, but Sen. Charles H. Percy (R), chairman of the Senate Foreign Relations Committee, was narrowly trailing Rep. Paul Simon (D). With ballots still to be tallied from downstate, where each man had strength, both men remained cautiously optimistic. Percy moved well to the right in seeking his fourth Senate term against Simon, a downstater and four-term congressman.

In Illinois House races, freshman Democrat Dick Durbin won a new term in the 20th District, while fellow freshman Democrat Lane Evans faced a stiff challenge from arch-conservative Kenneth G. McMillan.

Rep. Daniel B. Crane (R), who would have been a shoo-in had he not been censured for having had sex with a female congressional page in 1980, was declared a loser by one televison network in his race with state Sen. Terry L. Bruce (D).

In Simon's Carbondale district, former representative Kenneth J. Gray (D), a flamboyant wheeler-dealer in pork-barrel projects who was formerly known as "The Prince of Pork," was in a tough battle with Randy Patchett (R), the Williamson County state's attorney. Indiana

Indiana, a major beneficiary of the economic recovery and a state with one of the best-run Republican organizations in the country, gave the GOP early, easy victories. Reagan quickly captured the state's 12 electoral votes and Gov. Robert D. Orr (R) defeated state Sen. W. Wayne Townsend (D).

The National Republican Congressional Committee set its eyes on two seats held by Democrats, and both contests were expected to be close.

Rep. Philip R. Sharp (D), first elected in the Watergate year of 1974, has always had to struggle to hold on to his seat, and this year was no exception. His challenger, Ken MacKenzie (R), was well-financed and campaigned as a strong supporter of the president. Sharp won reelection.

However, freshman Rep. Frank McCloskey (D) was reported the loser in a contest with state Rep. Richard D. McIntyre (R), a conservative on economic and social issues.

Against a weak candidate in 1982, Rep. John Hiler (R) won with 51.2 percent of the vote, prompting Democrats to target the South Bend district this year. Hiler, a staunch Reagan backer, has benefited from the economic recovery and defeated Michael P. Barnes (D), a local prosecutor. Iowa

After Minnesota, Iowa offered Mondale his strongest chance of winning a Midwest state, but Reagan pulled ahead steadily as the night wore on.

The election results appeared to confirm earlier polls giving Reagan a 10-point edge in Iowa, with its eight electoral votes. The state is usually highly competitive.

In the Senate contest, Rep. Tom Harkin (D) beat Sen. Roger W. Jepsen (R), who had been widely viewed as the most vulnerable GOP incumbent. Jepsen, one of the most controversial members of the Senate, at home had been a member of a "club" cited for prostitution and in Virginia had claimed a senatorial right to drive alone in a special lane for cars with four passengers.

In Harkin's southwest Iowa congressional district, Jim Ross Lightfoot (R), a former radio announcer, was in a close race with Jerome D. Fitzgerald (D), a former majority leader in the state House. The outcome was expected to be a test of Reagan's support in a district heavily dependent on farming, which has been suffering from high interest rates and a drought.

In the northwest district now held by five-term Rep. Berkley Bedell (D), Darrel Rensink (R), the Sioux City manager, sought to portray Bedell as a closet liberal who casts himself as a moderate at home and votes for spending programs in Washington. Bedell held on to his seat, however. Kansas

In Kansas, with seven electoral votes, the only preelection question was the size of Reagan's victory. Polls had showed him with massive leads and he was declared an early winner. Sen. Nancy Landon Kassebaum (R), the popular daughter of 1936 GOP presidential candidate Alf Landon, easily won election to a second term over investment counselor James R. Maher (D).

Republicans successfully retained the 3rd District seat held by retiring Rep. Larry Winn Jr. as state Rep. Jan Meyers (R) defeated John E. Reardon (D), mayor of Kansas City.

Incumbents in the state's other four districts, evenly divided between the parties, were favored. In the huge 1st District, Rep. Pat Roberts (R) beat back a spirited challenge from a bankrupt farmer and rural activist, Darrell Ringer (D), who charged that Reagan's policies had been a serious burden for the state's wheat farmers. Michigan

Turnout in Michigan was unusually high as voters were splitting their ballots in the two major races. In the battle for the state's 20 electoral votes, Reagan decisively beat Mondale, despite Michigan's having one of the highest unemployment rates in the country. At the same time, however, liberal Sen. Carl Levin (D) was projected by two television networks to be headed toward a second term in his race against former astronaut Jack Lousma (R).

Two Democratic House members faced tough challenges, and the outcome there was seen as a test of the strength of Reagan's coattails in the state. Rep. Donald J. Albosta (D) lost to Bill Schuette, a well-financed Republican, in the 10th District, and Rep. Bob Carr (D), who lost in 1980 and regained his seat in 1982, had a strong lead over Tom Ritter (R), a farm market president, with about half the vote counted.

In the Grand Rapids district of retiring Rep. Harold S. Sawyer (R), the GOP nominee, state Sen. Paul B. Henry, won, despite an effective campaign by Gary J. McInerney (D). In two other competitive races, Rep. Howard Wolpe (D) held a slim lead over Jackie McGregor (R), and Rep. Robert W. Davis (R) was expected to beat Tom Stewart (D). Minnesota

Mondale came home to this state to vote yesterday, but with half the vote counted, he held only a slim lead over Reagan. The president, seeking a 50-state sweep, made an unscheduled stop in Minnesota last weekend in an effort to grab the state's 10 electoral votes.

In the Senate contest, freshman incumbent Rudy Boschwitz (R) capitalized on a 4-to-1 cash advantage to defeat Joan Anderson Growe (D), the secretary of state, by a wide margin.

Partisan politics in Minnesota are highly competitive and there were three potentially close House races, although the incumbents were favored in each.

Two Democratic freshmen, Timothy J. Penny and Gerry Sikorski, sought reelection. Penny was far ahead of Keith Spicer (R), a sales manager; and Sikorski, who won the suburban Twin Cities 6th District in 1982 with 50.8 percent of the vote, was leading attorney Patrick Trueman (R).

Rep. Arlan Stangeland (R), who has won by the skin of his teeth in three elections, was favored to win a fourth term despite the efforts of state Sen. Collin C. Peterson (D). Missouri

The president easily won Missouri's 11 electoral votes, and Attorney General John Ashcroft (R) defeated Lt. Gov. Kenneth J. Rothman (D) in the contest to succeed retiring Gov. Christopher S. Bond (R). Ashcroft, who showed unexpected strength in sweeping the GOP primary, got about 60 percent of the vote.

Another closely watched race involved state Sen. Harriet Woods (D), who became a national name in a 1982 bid to unseat Sen. John C. Danforth (R). Woods was in a close fight for lieutenant governor against Mel Hancock (R).

GOP strategists were counting on gains in the House, targeting Democrats Harold L. Volkmer in the 9th District and Robert A. Young in the 2nd. Volkmer, a conservative in a rural, Democratic district, was well ahead of Carrie Francke (R), a former assistant state attorney general. Francke received strong support from Danforth, for whom she served as 1982 campaign co-chairman.

The popular Young, from the St. Louis suburbs where ticket-splitting is common, was in a tight race against John Buechner (R), a well-known state representative. Incumbents (four Democrats, three Republicans) in the state's seven other districts were favored to win reelection. Nebraska

Nebraska, with just five electoral votes, was among the states competing to give Reagan his largest popular margin. Preelection polls showed Mondale with as little as 25 percent of the vote, and early returns showed Reagan carrying the state decisively.

The weakness at the top of the Democratic ticket presented the only danger to the reelection of Sen. J. James Exon, a conservative Democrat, but he easily defeated Nancy Hoch (R), a regent at the University of Nebraska, who was never close to Exon in the polls. The GOP had given her strong financial backing.

In an interesting House race, Thomas F. Cavanaugh (D) unsuccessfully attempted to gain the seat vacated by his brother, John, in 1980. The incumbent, Rep. Hal Daub (R), won a third term in the Omaha district by nearly a 2-to-1 ratio. North Dakota

North Dakotans put aside their state's farm problems and decisively gave Reagan their three electoral votes; it was the sixth time since 1960 that the state has voted for the GOP nominee. Gov. Allen I. Olson (R) was in early trouble in a race for reelection, trailing George Sinner (D), a state representative, after an intensely bitter campaign.

Rep. Byron L. Dorgan (D) defeated Lois Ivers Altenburg (R). Ohio

Democrats had high hopes in Ohio, where a Democratic gubernatorial administration liberalized voter-registration laws and significantly increased the electorate, but Reagan easily won the state's 23 electoral votes.

There were four competitive House races, involving three Democratic incumbents and one Republican.

The most vulnerable was Rep. Edward F. Feighan (D), a liberal freshman targeted for defeat by the National Republican Congressional Committee, which pulled out the stops for Matthew J. Hatchadorian (R). Feighan kept his seat, however.

In the past three elections, Democrats have gone after Rep. Lyle Williams (R) in the heavily Democratic Youngstown district. James A. Traficant Jr. (D), the flamboyant sheriff of Mahoning County, built a strong lead over Williams, signaling a likely upset.

The two other close races were an unsuccessful attempt by Frank Venner (R) to defeat freshman Rep. Marcy Kaptur (D) in Toledo, and the successful effort of four-term incumbent Thomas A. Luken (D) to hold on to his Cincinnati district against a challenge from Norman A. Murdock (R). South Dakota

Reagan, heavily favored from the start, easily won South Dakota's three electoral votes. Another easy winner was Sen. Larry Pressler (R), in a combative race against George V. Cunningham, formerly an aide to then-Sen. George McGovern.

In the battle for the state's only House seat, Rep. Thomas A. Daschle (D), a critic of Reagan farm policy, moved to an early lead in a tight race against Dale Bell (R), a steak-house operator from Spearfish. Wisconsin

Wisconsin went to the polls with virtually every opinion poll giving Reagan a big edge and that was how it turned out -- he swept the state's 11 electoral votes in a walk. All nine congressional incumbents were favored to win reelection.

In the fight for House seats, the biggest spending occurred in the 1st District, where seven-term Rep. Les Aspin (D) was challenged by Pete Jansson (R) of Racine. They raised more than $500,000 between them, but Jansson outspent Aspin by about $100,000.

Milwaukee's 4th District featured a rematch between Rep. Gerald D. Kleczka (D), who was elected in April to replace the late Clement J. Zablocki (D), and Robert V. Nolan (R), a dentist. Kleczka was favored.