Following is a state-by-state roundup of the Midwest (numbers in parentheses are Electoral College votes): Illinois (24)

This has been the toughest of the "Big Three" Midwest states for President Reagan to win, with the expanded black vote in Chicago and serious farm and farm-equipment problems downstate. But even before his weekend visits to Springfield and Chicago, the Post-ABC poll had him up 54 to 40 percent. Two polls in the state last week put the margins at 15 and 16 points.

The Post-ABC poll gave Sen. Charles H. Percy (R) a 49 to 40 percent lead over Rep. Paul Simon (D) in his fourth-term race, but tracking polls by both parties showed them dead-even all last week. The slam-bang contest has tarnished both men's gentlemanly image, and if the contest is as close as Illinois politicians think, the straight-ticket "Punch-10" vote-turnout efforts of rival Chicago Democratic factions headed by Mayor Harold Washington and Alderman Edward R. Vrdolyak could help Simon as much as the two late Reagan visits help Percy.

In key House races, Rep. Daniel B. Crane (R), censured by the House for sexual misconduct with a female House page, is struggling to hold off state Sen. Terry L. Bruce (D). Freshman Rep. Lane Evans (D) of Rock Island appears to be a step ahead in his rematch with former state senator Kenneth G. McMillan (R), and freshman Rep. Dick Durbin (D) of Springfield has a tough opponent in Sangamon County Board Chairman Richard G. Austin (R), but may come back.

Former representative Kenneth J. Gray (D), celebrated for his taste in public-works projects and other creature comforts during his 20 years in Congress that ended with a voluntary retirement in 1974, is favored over Williamson County State's Attorney Randy Patchett (R) for Simon's old House seat.

Former state senator Harris W. Fawell (R) should have no trouble succeeding retiring Rep. John N. Erlenborn (R) and state Sen. John E. Grotberg is a cinch to succeed Rep. Tom Corcoran (R), who unsuccessfully challenged Percy in the primary. Indiana (12)

Reagan is a runaway favorite in a state which has gone Democratic only once since 1936. Economic recovery has almost halved unemployment from the 1982 peak, helping Gov. Robert D. Orr (R) toward a second-term victory over state Sen. W. Wayne Townsend (D).

That makes it very tough for freshman Rep. Frank McCloskey (D) to hold his marginal district against state Rep. Richard D. McIntyre (R), who is helped by the strong GOP organization in Orr's home territory. McCloskey is as vulnerable as any Democratic incumbent in the country.

Rep. Philip R. Sharp (D), a five-term veteran of tough races, is favored over Ball Corp. executive Ken MacKenzie (R). Peter Visclosky (D) should have no trouble winning the seat of Rep. Katie Hall (D), the incumbent he beat in the primary. Iowa (8)

It is no accident that Reagan, Democratic presidential nominee Walter F. Mondale and running mate Geraldine A. Ferraro all are visiting this relatively small state in the campaign's final days. Farm problems have kept Reagan from developing the kind of lead here he enjoys elsewhere, and late Republican polls show the state a tossup. The final Iowa Poll in today's Des Moines Register showed Mondale trailing by 55 to 45 percent.

Sen. Roger W. Jepsen (R), seeking a second term, ranks as the most endangered Republican incumbent in the country, with recent polls by both parties showing him significantly behind his challenger, Rep. Tom Harkin (D).

Jepsen has labeled his rival "an ultra-liberal," but Harkin fought back with an effective television ad labeling Jepsen "the big spender" in the race because of his support for expensive weapons systems. Jepsen, beset by personal controversies, got his more moderate Republican colleagues, Sen. Charles E. Grassley (R) and Rep. Jim Leach (R), to make endorsement commercials, and narrowed the race somewhat early this week. But the Iowa Poll shows him trailing 55 to 45 percent, and some GOP officials say he appears finished.

In Harkin's old House district, however, former radio announcer Jim Ross Lightfoot (R) is given a slight edge over former state house majority leader Jerome D. Fitzgerald (D). Kansas (7)

Reagan led Mondale 59 to 24 percent in the poll published last week by the Kansas City Times, but was still only second-best in the state, trailing Sen. Nancy Landon Kassebaum (R), who is up 72 to 14 percent over investment counselor James R. Maher (D) in her bid for a second term.

With state GOP Chairman Dave C. Owen saying it would be "a disaster" for the Republicans to lose the one seriously contested House race, the party has invested heavily and managed to make state Sen. Jan Meyers (R) a slight favorite over Kansas City Mayor John E. Reardon (D) to succeed retiring Rep. Larry Winn Jr. (R), who has represented this area of suburban Kansas City for 18 years. Michigan (20)

From all indications, Reagan has outflanked Mondale and his major backers, Detroit Mayor Coleman Young (D) and organized labor's leaders, and secured a strong lead in Michigan, which is riding the auto industry's recovery, but is still above the national average in unemployment.

The latest Post-ABC poll gave Reagan a 54 to 38 percent lead, and the Detroit News poll last week had his lead at 14 percentage points, with Reagan getting two-fifths of the union members.

Massive ticket-splitting is propelling liberal Sen. Carl Levin (D) to a second-term win over former astronaut Jack Lousma (R). The Post-ABC poll had it 53 to 35 percent for Levin, and the Detroit News had the margin at 28 points.

Republicans have better hopes for a coattail victory in the Midland area, where Rep. Donald J. Albosta (D), the investigator of the 1980 presidential debate-papers incident, faces wealthy attorney Bill Schuette (R). Albosta has won three terms by playing conservative populist, farm-oriented politics, but Schuette, bolstered by a Reagan visit Friday, has a chance.

Rep. Bob Carr (D), bounced out of Congress for one term by Reagan's coattails in 1980, is given good odds of surviving this year's challenge from businessman Tom Ritter (R).

In Grand Rapids, state Sen. Paul B. Henry (R) should pick up the seat of retiring Rep. Harold S. Sawyer (R), though his opponent, lawyer Gary J. McInerney (D), is regarded as an exceptionally able candidate. Minnesota (10)

Mondale carried his home state twice as a Senate candidate and twice running for vice president, so it is implausible he will lose it this year. But Reagan is dead-even in late polls by his own campaign and others.

Sen. Rudy Boschwitz (R) is favored for a second term over Secretary of State Joan Anderson Growe (D) in a gender-gap contest that was moving in the challenger's direction for a time, but now is clearly tipping to Boschwitz. He has outspent her almost 4 to 1, and late polls gave Boschwitz an almost 20-point lead.

Freshman Rep. Gerry Sikorski (D) is a narrow favorite over Patrick Trueman (R), an attorney and antiabortion activist. In two other districts with close races in 1982, Reps. Timothy J. Penny (D), another freshman, and Arlan Stangeland (R) may have a bit easier time. Missouri (11)

Out of step with the country only once in the last 80 years (1956), Missouri has consistently produced polls giving Reagan around 55 percent of the vote. The GOP is favored to hold the governorship of retiring Christopher S. (Kit) Bond (R). Attorney General John Ashcroft (R) has led Lt. Gov. Kenneth J. Rothman (D) throughout the race and late private polls show no wavering in his margin.

The St. Louis Post-Dispatch poll released Friday had Ashcroft leading 44 to 35 percent, but with 20 percent still undecided.

Republicans have high hopes of upsetting one veteran House Democrat and perhaps another. Carrie Francke (R), a former state assistant attorney general and aide to Sen. John C. Danforth (R), is running an aggressive, well-financed race against Rep. Harold L. Volkmer (D), a conservative farm-oriented member.

In the more Republican St. Louis County, Rep. Robert A. Young (D) faces former state representative Jack Buechner (R), but Young has shown he can appeal to the conservative suburbanites. North Dakota (3)

Reagan is leading Mondale by a 2 to 1 majority in the polls, and Rep. Byron L. Dorgan (D) is far ahead in his race for reelection.

The battle is over the state house, where Gov. Allen I. Olson (R), who drew criticism in his first term for expensive taste in office decorations and a proposed state plane, is engaged in a nasty television ad battle with state Rep. George Sinner (D).

A University of North Dakota poll released yesterday showed Olson leading, 49 to 41 percent. Nebraska (5)

With Mondale getting about 25 percent of the vote in two newspaper polls, there might be a potential problem for Sen. J. James Exon's (D) second-term bid. But the conservative incumbent had a 53 to 34 percent lead over his challenger, university regent Nancy Hoch (R), in Friday's Omaha World Herald poll, and she doesn't appear to be close enough to grab Reagan's coattails. Ohio (23)

Mondale has been in Ohio 10 times since the convention and Ferraro just as often, but they have never knocked Reagan out of the lead. The Post-ABC poll puts the spread at 55 to 37 percent, in line with private tracking studies.

Republicans are targeting three House races and will probably get at least one.

Reagan visited the most vulnerable district Friday, boosting former state representative Matthew J. Hatchadorian (R) against freshman Rep. Edward F. Feighan (D) in Cleveland's ethnic suburbs.

Another freshman, Rep. Marcy Kaptur (D) of Toledo, got into a long dispute with CBS over a campaign ad, which did not help her chances of withstanding television newscaster Frank Venner (R), who is seeking an upset in a Democratic district.

Rep. Thomas A. Luken (D) is controversial enough in the more Democratic part of Cincinnati to be potentially vulnerable to the challenge of Hamilton County Commissioner Norman A. Murdock (R).

Democrats have an outside chance in the depressed steel city of Youngstown, where Rep. Lyle Williams (R) faces colorful, controversial Mahoning County Sheriff James A. Traficant Jr. (D). South Dakota (3)

Although the farm economy is hurting, it is not being blamed on Reagan, who is headed for an easy win, or on Sen. Larry Pressler (R), who also is headed for a second term. George Cunningham (D), the former George McGovern aide who is challenging Pressler, points out that Pressler's brother, Dan, is in bankruptcy court, trying to save the Pressler family farm, and asks, "If he can't save his own farm, how do you expect him to save any others?" Even that pointed attack hasn't convinced people to vote for the Mondale-Cunningham ticket. Wisconsin (11)

The Badgers have persistently disappointed Mondale in his presidential campaign, from a state convention straw vote to their advisory primary, and they appear to have one more rebuff ready for their neighbor to the west. With the economy healthy, Reagan is running more than 20 points ahead in the latest Post-ABC and other polls, despite an active Democratic-labor campaign effort for Mondale.