A win, a loss for GOP senators highlight role of conservatives.

President Reagan's ''coattails'' were not quite long enough this year in Illinois. While Mr. Reagan got 57 percent of the state's votes, Sen. Charles H. Percy (R) garnered only 49 percent in his bid for a fourth term. His Democratic opponent, US Rep. Paul Simon, squeaked to victory with a slim margin of some 75, 000 votes.

Mr. Percy may have been helped significantly by Reagan's win nevertheless. His loss - a surprise to some - might have been greater without the popular President heading the ticket, observers say.

''In many respects, Percy probably did benefit from Reagan's coattails - but just not quite enough,'' says James Nowlan, a political science professor at the University of Illinois, who managed the 1978 Percy campaign.

The clearest signal comes from another statewide race - for trustees of the University of Illinois. This is an invisible contest, Professor Nowlan says, and a good indicator of party voting. The candidates are virtually unknown and voters only have the candidates' party affiliation to guide them. In the past, Nowlan says, these races have tended to reflect the state's slight leaning toward Democrats. In 1982, for example, Republicans on average got 43 percent of the vote (each party runs three candidates and the top three vote-getters are elected). But this year, the Republicans averaged 50 percent - about what Percy got in the senate race.

His failure to win may not be due so much to a lack of coattails, but to a host of local variables, Nowlan suggests.

For one thing, Percy's Senate record, especially as chairman of the Foreign Relations Committee, was weighed down with a lot of baggage, observers suggest. His approval of the sale of sophisticated radar aircraft to Saudi Arabia, for example, hurt his standing among Jewish voters, according to an NBC poll.

Although he tried to position himself as a Reagan convert, Percy's moderate voting record was hard to sell to conservative voters. ''They weren't all that gung-ho for him,'' says a Democratic township chairman in DuPage County, traditionally a GOP stronghold. ''He wasn't one of them.''

Part of Percy's support was siphoned off by Libertarian Party candidate Steve Givot, who received slightly more than 1 percent of the vote. But by themselves, neither the Jewish nor the Libertarian vote would have given Percy a victory.

The real trouble was Simon's attacks on Percy's effectiveness, says John Jackson, a political science professor at Southern Illinois University.

At one point Tuesday night, Percy strategists seemed confident of victory. They reportedly expected Simon to garner a big vote in heavily Democratic Chicago. But Percy strategists said they expected to win statewide, even if Percy won only 26 percent of the city's vote. They were buoyed he grabbed 30 percent.

But the big surprise was downstate, where Republicans normally win. Simon, a downstater himself, ran well this time. Percy strategists attributed this to several things. A $1 million anti-Percy ad campaign by a Los Angeles real-estate developer may have swung voters, they said. Rising state unemployment, reported just before the election, and layoffs in the region also played a part. Simon (D) 2,365,413 50.2% Percy (R) 2,290,587 48.6%

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