Why Percy vs. Simon is a cliffhanger

In Illinois, Sen. Charles H. Percy appears to be in the fight of his life - for reasons that to him must seem unfair. * Chicago's Mayor Harold Washington and his political organization are calling the signals. For the first time since Senator Percy ran for office, the black community seems bent on denying him a substantial vote.

Blacks have long honored President Kennedy, because they felt that he, and his brother Robert, stepped in early on their behalf. Similarly, in Illinois Mr. Percy has long been regarded as one of their champions since the 1950s and '60s, when he became a leader in forwarding civil rights, particularly in fair-employment practices.

Will Chicago's blacks remember that Percy was their friend when the going was tough? Political pressure from the black mayor is putting the answer in doubt.

* Much of the Illinois Jewish community was at one time in the Percy camp, seeing in him a leader with views they could support. That was before he came from a visit to the Mideast with the view that a solution be found for the Palestinian problem, one that would entail creating an entity for the Palestinians on the West Bank.

At the time it was falsely reported that Percy favored a deal with the Palestine Liberation Organization, perhaps even talks with Yasser Arafat before Arafat's acknowledgment of Israel's right to exist as a nation. Those who heard Percy's comments knew he never went beyond what both former President Carter and President Reagan have said in their search for a solution. Like them, Percy has always qualified his position with the assurance that he favored guaranteeing Israel's existence.

But many Illinois Jewish leaders have decided that Percy is their foe, someone who must be removed from his key role as chairman of the Senate Foreign Relations Committee. Their funds have been put into a zealous effort to unseat the man they once admired.

* The Republican right wing in Illinois, which has never been enthusiastic about the moderate Senator Percy, could be solidly behind him as he seeks a fourth term. Percy's record mirrors that of the President's programs. But support from the GOP conservative rank and file remains an imponderable for the veteran senator.

* Finally, there is Percy's opponent, Rep. Paul Simon, a potent adversary. Mr. Simon is perhaps the fairest of the fair-haired in Illinois Democratic politics.

Since Watergate the public's view that ''politics is dirty - and politicians are dirty'' seems to have widened and deepened. There is a feeling among the electorate that politics causes public servants to become less than ethical. I would argue that we have a remarkably high percentage of principled people in government. And as an example, there's Percy vs. Simon in Illinois.

Percy has always been ''Mr. Clean.'' And Simon, regarded as highly ethical when he served in the Illinois legislature and as lieutenant governor and now as a congressman, could well vie with Percy for that title.

Percy against Simon. Senate control could turn on the outcome. If Mr. Reagan wins, his coattails could help Percy. Reagan has already been to Illinois in Percy's behalf - and he will go again during the campaign.

The voters may decide that experience and Percy's position of influence in shaping foreign affairs is enough to keep him in office. But polls show that Simon's challenge is the strongest that Percy has ever faced.