Using $475,000 of his own money, Sen. Charles H. Percy (R-III.) is making one last run at Illinois voters this week with a daring and desperate plea.

The polls say many of you want to send me a message," Percy says earnestly in a hastily produced television commercial now showing all over the state. "But after Nov. 7, I may not be in the Senate any longer to receive it.

"Believe me, I've gotten the message, and you're right. Washington has gone overboard, and I'm sure I've made my share of mistakes.

"But in truth, your priorities are mine, too - stop the waste, cut the spending, cut the taxes."

It is the sort of plea that only a politician in deep trouble would even consider, and Percy - one of the few remaining liberal Republicans in the Senate - is in deep trouble. Running against a nominal Democrat named Alex Seith - a relative unknown who has put $750,000 of his wife's money into his slick media campaign - Percy is behind in some polls and almost deadlocked even in his own.

The latest statewide straw poll by the Chicago Sun-Times gives Seith a 54 per cent to 46 percent lead over Percy.

This Senate race seems to embody much of the confusion in American politics in this Proposition 13 election year. And it demonstrates unmistakably that the advantages of incumbency can be all but obliterated by a cleve - some here would say dishonestly clever - challenger.

In the closing days of the campaign, Seith and his media campaign have become Percy's principal issues. The senator charges that Seith's smooth commercials have misrepresented Percy's record and implied he was a racist. Many members of the Illinois establishment seem to agree.

All the major newspapers have endorsed Percy. The state's best-known journalist, columnist Mike Royko of the Chicago Sun Times, has written a series of columns lambasting Seith, one of which the Percy campaign reprinted this week as a full-page ad in every daily newspaper in the state. Muhammad Ali and the Rev. Jesse Jackson have rushed to Percy's defense, making radio spots on his behalf.

Numerous prominent Democrats say - some publicly, some privately - that they will vote for Percy Tuesday at least partly in protest against Seith's tactics. Seith's campaign manager, Gary South, dismisses these people as "a motley group," and Seith says many are motivated by class loyalty to Percy.

Seith is confident he can compensate for any lost Democratic votes by picking up Republican support, particularly in rural, downstate Illinois and the Chicago surburbs. In the Sun-Times straw poll, which has a reasonably good reputation for accuracy, Seith is leading Percy throughout the downstate region. The senator's moderate, internationalist voting record and his personal manner displease many conservative voters.

If one incident crystalized anti-Seith feeling in the media and Chicago establishment circles, it was a Seith radio commercial prepared by New York media consultant Tony Schwartz for broadcast on black-oriented radio stations.

The ad picks up on remarks by Percy in downstate discussionson agriculture. On two occasions Percy endorsed the free-market policies of agriculture secretary Earl Butz in the Nixon and Ford administrations and said he'd like to have Butz back on the job.

This is the Seith radio spot, read by an announcer:

"Do you think Sen. Percy is a friend of black people? Well, remember Earl Butz? He was that secretary of agriculture who made a racist and sexually obscene joke about blacks. We can't repeat his words on the air, of course, but they were so offensive that he had resign. Maybe you are wondering what that's got to do with Sen. Percy. Just this - Sen. Percy said of Earl Butz. . .'I wish he were secretary of agriculture still today.' Still today, Sen. Percy? Percy wants the black vote, and with friends like this, you don't need enemies. Because Charles Percy tolerates the Earl Butz insult to blacks, more and more people are getting behind Alex Seith for the United States Senate. . ."

Percy and many others, including several Chicago columnists, the Rev. Jackson and Ali, have charged that this ad depicts the senator as a racist. Percy now says angrily at virtually every campaign stop that he called for Butz's resignation the moment he learned of the Butz joke two years ago, and that he telephone Butz to tell him to step aside.

In a joint TV appearance Tuesday night, Seith acknowledged that Percy had deplored the Butz joke. He defended the radio spot by saying, "It doesn't imply that (Percy is a racist), and anyone who says so is not listening."

A TV reporter questioning Seith replied, "I'm listening, Mr. Seith, and I say it implies that." The riposte was typical of the harsh treatment Seith now receives in the media here.

Seith's positions on issues may be unprecedented in modern times for a Democratic candidate in a northern industrial state. In public appearances, advertising and private discussion, Seith endorses none of the traditional New Deal to Great Society prescriptions of the Democratic mainstream.

Instead, he favors and end to federal aid to education, coupled with something he calls "revenue keeping" that would allow states to retain 10 percent of the taxes their citizens pay to the federal government. These funds could then finance schools and other local programs, he says.

Seith's second major economic proposal is a "pay protector." In one of the masterful TV ads prepared for his campaign by David Sawyer and Associates of New York, Seith explains it this way:

"Here's the idea: as the cost of living goes up, taxes will go down."

In other words, taxes would be indexed to inflation, declining when inflation gets worse. Asked in an interview if these two program and other tax cuts he publicly favors would drastically reduce other revenues, Seith contended that they would stimulate new growth.

His economic positions, like much in his campaign, play to sentiments discovered in the Illinois electorate last summer in polls taken by Peter D. Hart Research Associates Inc. of Washington. Hart found that Percy had extremely thin and unenthusiastic support for a two-term incumbent, and identified voters who called themselves conservatives as particularly disinclined to support the senator this year.

One Seith radio commercial includes a statement by a man who describes himself as a lifelong Republican who plans to vote for Seith this year because his proposals look "a lot better to me as a conservative than the performance of Alex's opponent, Sen. Percy."

Seith has proven an adept campaigner. He has traveled the state knocking on voters' doors, establishing a man-of-the-people image, but his best moments come on television.

Seith has what he describes as 'a great face for Illinois politics" because "Greeks think I'm Greek, Jews think I'm Jewish," etectera. (In fact, he's a mixture of German, Hungarian and Scottish blood.) Seith has a smooth, pleasing voice, an easy, winning manner and a great way with words. People who have known him as a corporate lawyer and president of the Chicago Council on Foreign Relations describe him as a complex personality with a first-rate brain.

By contrast, Percy is long-winded, less articulate and apparently more likely to impress ordinary citizens as vaguely pompous or haughty. "He's not perfect," Percy's campaign manager acknowledges gamely.

Though both men are wealthy - Seith by dint of his marriage to a Rochester, N.Y., heiress and Percy through his long career at Bell and Howell - Percy comes across as a patrician, Seith as more accessible. Seith is a graduate of Yale and the Harvard law school. He grew up in a middle-class family in Aurora, Ill, and appears in many of his TV ads in shirtsleeves and slacks.

Percy will spend about $1.5 million on the campaign, Seith roughly $1 million.

Percy's biggest asset appears to be his reputation as a serious senator who gets visibly involved in big issues. In his campaign appearances Percy repeatedly tells audiences how "Ted Kennedy and I" worked for the 18-year-old vote, or "Ed Muskie and I" wrote the new congressional budget legislation. He reminds people that he fought President Nixon's controversial Supreme Court nominations or struggled to end the Vietnam war.

He sounds senatorial.

But his campaign hasn't been working. "I can't be dishonest with myself," he said in an interview. "The facts are there. There's been a tremendous erosion in my support."

So in the closing days he has tried to focus attention on Seith and his campaign advertising, blaming his opponent's "negative campaign" - which Percy called untruthful - for the dramatic change in the polls.

One reasonably scientific poll conducted by the CBS affiliate in Chicago found that Percy lost 17 points in the second half of October. In the Sun-Times straw poll as of Wednesday. Seith led Percy 57-43. Percy's own poll, completed Tuesday by Robert Teeter of Detroit, showed the race too close to call.