Chicago — The tan and red twin-prop plane landed at Chicago's waterfront Meigs Field at the tail end of a downpour and 15 minutes behind schedule. Dozens of children on an outing from a day-care center clapped and cheered as the man they'd been told was a celebrity - dressed in red bow tie and navy-blue business suit - climbed out.
Rep. Paul Simon (D) of Illinois hastened to a second-floor airport meeting room and, amid a crush of waiting reporters and photographers, began the announcement everyone was expecting: ''I am a candidate for the US Senate. . . .'' He was to make it seven more times in various Illinois cities that same day.
In these days of hot weather and hot politics in Illinois - and the 1984 election is still more than a year away - Senate candidates in this state have been proliferating with all the vigor of freshly watered spring flowers. They are after a coveted political prize - the seat now held by Illinois Republican Charles H. Percy, who prepares to seek his fourth term.
For the moment, since the issues are not yet defined and endorsements are sparse, the site and timing of campaign announcements have been netting much of the attention. Just one week ago Illinois State Comptroller Roland Burris (D), the only black so far in the race, announced his Senate candidacy beside a statue of Abraham Lincoln in Chicago's Grant Park. Just a month earlier, from the steps of an Ottawa, Ill., mansion where many Illinoisans once stood to watch the 1858 Lincoln-Douglas debate, conservative Republican Rep. Tom Corcoran announced his run.
As presidential campaigns seem to stretch out longer and longer, so do races for Congress. ''As campaigns get more and more expensive, this is one very direct result,'' concedes Democratic National Committee spokesman Robert Neuman.
Though much of the early announcing is aimed at inducing organizational and dollar support, there's also the hope that preemptive action will ward off more of those candidates who may be thinking of running but have not yet cast the die. Mr. Simon, a five-term Congressman who formally entered the Senate race Monday, said he doubted that all candidates mulling a run would make it official.
So far, three Democrats have declared and il20l,0,13l,5panother three or four , including Illinois Democratic State Central Committee chairman Philip Rock, are weighing the race.
Senator Percy's Illinois Senate seat is one of four (along with North Carolina, Iowa, and Tennessee) which, political analysts say, Democrats must win if they are to recapture a Senate majority. In 1984, 19 Senate Republicans are up for reelection, compared to only 12 Democrats.
''Usually it's the Democrats who are risking more, but this time it's the Republicans who are in the most vulnerable position,'' says John Jackson, a professor of political science at Southern Illinois University. ''The Illinois race is competitive in that either side could win, but it's also pivotal in that it could be absolutely crucial for the Democrats in terms of controlling the Senate.''
Though Percy faces a tough March '84 primary challenge this time from Mr. Corcoran, many political analysts argue that the senator may emerge from it in a much strengthened position for the November election fight if he triumphs.
Corcoran argues, for instance, that Percy, as chairman of the Senate Foreign Relations Committee, has obstructed President Reagan's foreign policy and votes more like a Democrat than a Republican. The conservative Committee for the Survival of a Free Congress sent out letters as long ago as last fall to raise money to unseat the senator, who it labels ''ultraliberal.'' And a national anti-abortion group plans a major campaign effort to defeat him.
If Percy can weather these conservative attacks, it is assumed that the charges from Democrats, such as Mr. Simon, who argues that Percy has taken a ''turn to the right'' and has become the ''flagbearer'' of Reagan economic policies, would carry less weight.
But no one, least of all Senator Percy, is assuming victory. He has been visiting Illinois much more frequently since his narrow win in 1978 over Democratic challenger Alex Seith, and has raised $1 million toward the 1984 campaign. A survey conducted for Simon by Washington pollster Peter Hart shows Simon with a strong lead over Democratic contenders and behind Percy by six points.
''Illinois is always a state which has closely contested elections,'' notes Republican National Committee spokesman William Greener. ''We are certainly not taking anything for granted.''