The September Chicago Tribune poll on the Illinois Senate race showed Rep. Paul Simon (D) leading Sen. Charles H. Percy (R), 42 to 40 percent, not the reverse, as reported yesterday. A report Oct. 5 on the results of a Washington Post-ABC News presidential preference poll included incorrect findings for California. The article should have said President Regan led Walter F. Mondale there by 51 to 40 percent, not 49 to 43 percent. The error resulted from adjustment of the polling sample to match Census Bureau figures for key characteristics of the state's population. Such adjustments, known as "weighting," are commonly used in polling. In this instance, the Census Bureau provided incorrect figures.

Heading into Sunday's presidential debate, President Reagan has a lead over Walter F. Mondale broad enough to make possible an electoral-college landslide and deep enough to boost the hopes of Republican candidates for other offices, the latest Washington Post-ABC News poll shows.

The Sept. 22-Oct. 2 poll is a super-sized sample of 12,000 people that is designed to measure the presidential race state by state and nationwide. It also surveyed individual races for the Senate and governorships.

Nationally, Reagan led his Democratic challenger 55 percent to 37 percent. The margin is up slightly from his 16-point, or 56-to-40 percent, lead in a poll taken Sept. 6-11.

But the breadth of his lead was shown more clearly by the state-by-state polls. Reagan led everywhere except the District of Columbia and Rhode Island. In Rhode Island, he and Mondale are even. California and New York were the only other states where Reagan's lead was less than 10 points.

The survey also showed that the Democrats made little headway in the first month of the fall campaign. The personal ratings of Mondale and his running mate, Rep. Geraldine A. Ferraro (D-N.Y.), have slipped further since the early September poll, while those of Reagan and Vice President Bush have improved slightly.

Moreover, the GOP has widened its lead over the Democrats as the party voters trust to do a better job in coping with the main problems facing the nation. Last May, the Democrats led 42 percent to 40 percent on that measure. By early September, the Republicans were favored, 49 percent to 39 percent. In the new poll, the Republicans have a 47-to-33 percent lead -- their largest so far.

In Senate races, the individual state samples suggest that Democrats are in position to challenge for Republican-held seats in at least four states. But Republicans, who once feared that their 55-to-45 Senate majority might be almost eliminated, may now be able to keep their losses to a minimum.

Democrats are far ahead in Tennessee, slightly ahead in North Carolina and narrowly behind in Illinois and Iowa. The only Democratic-held seat that looks promising to the GOP is in Massachusetts, but the Democrats are slightly ahead there now.

The Post-ABC News poll did not ask voters for their choice in the House of Representatives. But a Market Opinion Research survey for the National Republican Congressional Committee that came out of the field the same day the Post-ABC poll began and had a virtually identical 47-to 32 percent result on the party confidence question, showed a very narrow margin for the Democrats on the congressional vote.

In the NRCC poll, respondents said they planned to support Democratic candidates over Republicans 47 percent to 44 percent. That was a drop from a 50-to-41 percent Democratic edge in an August GOP poll and a 51-to-43 percent edge in the early September Post-ABC poll.

Continuation of that trend could set the stage for Republican gains in the November House elections.

The gubernatorial battles, according to the Post-ABC Poll, look a bit better for the Democrats. There could be several turnovers, with Republicans having opportunities to pick up governorships in West Virginia, Utah and Rhode Island, and with Democrats' prospects almost as good in Washington, Missouri and Vermont.

With almost 12,000 registered voters interviewed in the Post-ABC poll, the survey is one of the largest, if not the largest, ever done before a presidential election.

In effect, the overall survey consists of separate polls in each of the 48 contiguous states, with samples ranging from about 800 in California and New York, down to about 150 in the smaller states. The sampling error ranges from below 4 points to about 8 points, depending on the number of interviews in each state.

Some interviewing also was done with residents of Alaska, Hawaii and the District of Columbia, but not enough to be used for statistical models. As might be expected, almost all the D.C. residents interviewed said they were supporting Mondale.

Elsewhere, however, the results looked mostly bleak for the former vice president, who is hoping that his Sunday night television debate with Reagan in Louisville will light fires for a comeback drive. But no challenger has ever overcome the odds Mondale faces in this poll.

The picture is dramatically sketched in the big states, where most elections are decided. Mondale trails in every one of them, coming closest in this poll in the two giants. He is running 6 points behind Reagan in California and 7 points behind in New York.

From there, it gets worse. Reagan's margin is 11 points in Ohio, 13 in Pennsylvania, 14 in Michigan, 17 in Illinois, 18 in North Carolina, 20 in Texas, 21 in New Jersey and 26 in Florida.

These numbers make clear that Mondale must try to move the electorate nationally; no targeted tactic aimed at a small number of states or voters offers him hope of success in the final month. Reagan, on the other hand, has such a seemingly secure base that he can concentrate on those few states that are, by a generous definition, battlegrounds today.

Mondale's task is complicated by the fact that, on several basic measures, he and Ferraro seem to be moving backward. Mondale's favorable rating has slipped from 47 percent in early September to 41 percent now, while his unfavorable rating has stayed at 49 percent.

Ferraro has dropped from a 50 percent positive-39 percent negative rating to a shaky 44-to-42 percent favorable.

By contrast, Reagan and Bush are holding at 60 percent favorable, with the negatives for each of them dropping 3 or 4 points, to 38 percent for Reagan and 28 percent for Bush.

Only one-third of those surveyed think Mondale has strong leadership qualities, while two-thirds see those qualities in Reagan. Reagan leads Mondale in each of the issue areas surveyed.

Here are the Post-ABC survey findings and other recent polls on some of the states with key presidential or Senate races.

California. Reagan has never lost in his home state, but Mondale and Ferraro have narrowed his margin by making two visits each. An early September Field poll put the margin at 16 points; a KABC-Teichner poll in the third week of September put it at 12 points, and the new Post-ABC poll has it at 49 to 43 percent for Reagan.

Illinois. The 17-point Reagan margin in this poll contrasts with a 10-point Reagan edge in a slightly earlier Chicago Tribune poll. Sen. Charles H. Percy (R) led Rep. Paul Simon (D) 42 to 40 percent in the Tribune poll and 46 to 39 percent in this one, indicating that the race is in the hands of the undecided voters.

Iowa. Mondale trails in the poll by a significant margin, but the contest between Sen. Roger Jepsen (R) and Rep. Tom Harkin (D) looks closer, both in this poll and in one taken earlier by the Des Moines Register, which gave Jepsen a 9-point lead.

Massachusetts. Reagan had a slightly bigger lead in this poll than the 8-point margin in an earlier Boston Globe poll. In the race for the seat of retiring Sen. Paul Tsongas (D), Lt. Gov. John Kerry (D) led businessman Raymond Shamie (R) by a margin almost identical to the 47-to-40 percent lead reported in the Globe poll.

Michigan. Reagan leads 52 to 38 percent in this poll, compared with 54 to 37 percent in a slightly earlier Detroit Free Press survey. But Sen. Carl Levin (D) has a 22-point edge on former astronaut Jack Lousma (R) in this poll and a 20-point margin in the Free Press Poll.

New York. The 49-to-42 percent Reagan edge in this poll is tighter than the 14-point margin he enjoyed in The New York Daily News poll earlier in September.

North Carolina. Reagan is winning handily, but Gov. James B. Hunt Jr. (D) leads Sen. Jesse Helms (R) 51 to 42 percent in this poll. State polls have shown the lead bouncing back and forth but recently have been more favorable to Helms.

Ohio. The 51-to-40 percent Reagan margin in this poll may suggest some narrowing. Two mid-September polls had the gap at 17 and 15 points.

Pennsylvania. Reagan has a 52-to-39 percent margin in this poll, but there is no other recently published data with which to measure a trend.

Texas. Reagan's 57-to-37 percent lead in the Post-ABC poll is less than two mid-September polls taken by others which showed Reagan leading by 29 and 26 points. In the race for retiring Sen. John Tower's (R) seat, Rep. Phil Gramm (R) led Lloyd Doggett (D), 49 to 37 percent in this poll, about the same margin as in earlier state polls.

The likeliest Democratic Senate pickup continues to be the Tennessee race, where Rep. Albert Gore Jr. (D) runs far out in front for the seat Senate Majority Leader Howard H. Baker Jr. (R) is vacating.

In the gubernatorial races, the best bet for a Republican gain is in West Virginia, where former governor Arch Moore (R) is well in front in the race to succeed Gov. John D. Rockefeller IV, (D), who in turn is leading in the race to succeed retiring Sen. Jennings Randolph (D). The best Democratic chance appears to be in Washington, where Pierce County Executive Bruce Gardner (D) came out of the September primary with a strong early lead over Gov. John Spellman (R).