Tinley Park, Ill. — The Fulton Commons Go-Getters were getting restless. The senior citizen group's business meeting had long since ended, and the speaker of the day - incumbent US Sen. Charles H. Percy (R) of Illinois - was running 45 minutes late.
When the senator finally arrived, he was all apologies (he had followed a long-winded commencement speaker at the last stop, he said) and was determined not to disappoint. He introduced his star-studded supporting cast: wife Lorraine and two of the most prominent Democrats for Percy - daughter Sharon Percy Rockefeller and her son Justin. By the time the senator hit the road again less than half an hour later, he had carefully polled the Go-Getters on such issues as eliminating the mandatory retirement age, shaken hands warmly all around, sat down at a piano to play a little ''polluted'' Rachmaninoff (''That's enough,'' said his wife with a smile after a few bars), and helped cut the group's party cake.
Elections are still four months off. But Percy's Senate seat has long been targeted nationally by Republicans and Democrats as a pivotal one to keep, or capture, if the GOP's five-seat US Senate majority is to be held, or broken. And both Senator Percy, who seeks a fourth term, and Rep. Paul Simon (D), a five-term congressman and former Illinois lieutenant governor, are making the July rounds of picnics, parades, and baseball games with an intensity that suggests the election results depend on it.
Both men have learned to take nothing for granted.
Representative Simon, whose parents were Lutheran missionaries and whose preference for bow ties has become a personal trademark, lost the Democratic gubernatorial primary here in 1972. But he emerged the decisive winner this year in a four-way primary fight for the right to challenge Percy.
The senator, who rose to the presidency of Bell & Howell at 29, won with only 53 percent of the vote in his 1978 race against suburban Chicago lawyer Alex Seith. Since then he has visited all 102 Illinois counties and returns to the state to campaign almost every weekend. And although pollster Robert Teeter's surveys show the senator leading his opponent, Percy expects a ''photo finish,'' his press secretary, Kathy Lydon, says.
The current race marks a new challenge for Percy, who was the strong winner in a tough primary fight with conservative GOP Rep. Tom Corcoran, now a Percy supporter. Simon is decidedly more liberal than most of Percy's past Democratic opponents. Accordingly, Simon has laid claim to some of the endorsements from labor, Jewish, and independent voter groups once found in Percy's camp.
Political analysts often describe both men as relatively liberal. But in separate interviews, the candidates insist they are fiscal conservatives but are particularly far apart on economic issues.
Percy, endorsed by the US Chamber of Commerce and the National Federation of Independent Business, supported President Reagan's 1981 economic recovery program with its tax cuts. And he favored the White House-proposed inheritance tax reductions, which he says ''saved'' the family farm and small business.
Simon opposes Reagan's economic policies. He says they have primarily helped the very wealthy and hurt Illinoisans.
On national security, Simon says he opposes the MX missile, which Percy supports, and is a co-sponsor of the nuclear freeze resolution, which Percy opposes. A co-sponsor of the bill to move the US Embassy in Israel from Tel Aviv to Jerusalem, Simon has accused his opponent of being a fair-weather friend of Israel, supporting it most strongly only during election years.
''He has distorted my position,'' insists Percy, who says his support of Israel has been strong and steady.
Simon also accuses Percy of being changeable, noting that the Baron Report, a political newsletter, described the senator as having a more conservative voting record in some areas than Sen. Jesse Helms (R) of North Carolina. And attacking the incumbent's campaign theme - ''the Illinois Advantage'' - the congressman charged that Percy has been a less-than-effective senator. Simon insists he himself has sponsored more bills to pass the House this year than any other member.
Percy has long described himself as a progressive Republican and says he has changed his position only as progress and changed circumstances require it. He says he has supported Reagan on issues about 80 percent of the time. Noting that Sen. Edward M. Kennedy (D) of Massachusetts has supported the President about one-third of the time since 1980, Percy says Simon's record of Reagan support over the last year has been closer to 15 percent.
Percy says his opponent ''has tried to paint me as an ultraconservative and flip-flopping. . . . I'm just answering in kind, if that's the game he wants to play. I'll call him what his record proves he is - ultraliberal - certainly to the left of Teddy Kennedy.''
Percy insists his position as chairman of the Senate Foreign Relations Committee is an advantage to Illinois as an export state and that polls show voters here perceive it as such. Though conservative Senator Helms is next in line for the top committee post if Percy loses, Helms is locked in a tight re-election campaign of his own and would have to choose between that job and his chairmanship of the Senate Agriculture Committee (considered more important to voters of his state), even if Republicans should keep a majority.
Simon, who says his own district is south of Richmond, Va., and ''Bible Belt conservative,'' says he expects to garner a number of conservative votes statewide and to do particularly well among black, Hispanic, and independent voters. He has already bridged the Chicago Democratic gap to win the support of Mayor Harold Washington and Cook County Democratic chairman Edward Vrdolyak.
''The only problem for us (Republicans) in Illinois is the city of Chicago - that's our weak link,'' says GOP State Central Committee chairman Don Adams. Republican strength downstate is stronger than it has been in some time, he says , and the party hopes to register 100,000 new voters by fall.
In the end, much may depend on Reagan's popularity here. It may be worth noting that in 1980, when 21 of the 22 counties then in Simon's district supported Reagan, Simon's margin of victory shrank from a usual high of more than 50,000 votes to about 2,000.