As Republicans convened in Detroit this week to draw their blueprint for economic prosperity, the building they want to remodel started shaking in Peoria.

For 20 years, through national slump and recession, the Caterpillar Tractor Co., largest private employer in Illinois, was untouched -- employment held steady, sales and profits soared.

Then last week, the first layoffs in two decades hit the four big Caterpillar plants in the Peoria-East Peoria area. Rumors of more to come, reaching perhaps many thousands, have made this Corn Belt industrial center uneasy and tense.

Yet this traditional Republican stronghold evinced little excitement for the GOP convention in Detroit and the upcoming presidential campaign despite Ronald Reagan's promises to improve the economy.

Here, that's the issue, for as Caterpillar goes, so goes Peoria. Cat, as it's called here, employs about one of every 10 workers in this metropolitan area of 330,000 people.

Caterpillar's 35,000 employes are on their annual two-week vacations. The plants are closed, idle storage yards hold row after row of big yellow earth-moving machines. Outside the plants, at usually busy bars like the Glee Club, a blue-collar oasis on the edge of the East Peoria complex, business drags.

Caterpillar plant workers aren't paying much attention to politics, the central view being that the Republican convention was too cut and dried to merit watching on television. Management types seem nearly as blase, although Reagan seems their choice to turn things around.

The threat of layoffs and possible hard times ahead has kept many like Arthur Thomas, 22, a diesel engine assembler, vacationing close to home, counting his pennies.

"I lost almost $7,000 due to our three-month strike last fall and I'm saving now because if I'm laid off, it might be for more than a year," Thomas said. "I don't see much to stop this, although I think John Anderson could get the country back together. Jimmy Carter is what he sells -- a peanut."

But Art Thomas, like many of his buddies from the production line, isn't focusing much on the campaigns or conventions.

"I'm a little uneasy," said Wayne Schmidt, recording secretary of Local 974 of the United Auto Workers. "It scares me. Anderson looms as the unknown in this election. And a lot of people, like my brother, are Democrats who seem to be for Reagan."

"But there is a lot of political apathy here. People are grasping, trying to get a hold on the everyday economy. Buying a home is almost impossible for our younger workers."

With 23,000 members, Local 974 is the second largest in the UAW. But unlike the big UAW units in Detroit and other auto manufacturing centers, Local 974 members rarely have faced the unhappy prospect of cutbacks and layoffs.

Now, the deepening recession and the worldwide economic slump are bringing reality to Peoria.

About 1,100 Caterpillar workers were laid off last week, most of them eligible for unemployment benefits amounting to about 95 percent of their wage, which averages $11 an hour.

"Our industry has not been subject to the pressure from imports that has hurt the auto industry," said Jim McGuire, education director for Local 974. "And Caterpillar is one of the best managed companies in the country. Our problems are due to the national situation."

"Yet," he continued, "we're looking at our first layoffs in 20 years. We feel we're only going through a slump. Some people still are working overtime while others are being laid off. The company has reached a point where it is cutting out the fat, and those are the jobs that are going now."

McGuire repeats a customary line of organized labor -- not enthralled with Jimmy Carter as president, but even less enthused about Reagan and the Republicans.

"I think our workers are looking toward the lesser of two evils, looking toward Carter with reluctance," McGuire said.

One of those reluctant supporters is William Allen, a 12-year employe who works as a planning analyst. "You can't blame one man for the situation we're in. It took a while to get into this and there is no overnight cure. It doesn't matter who gets in -- it will be four or five years before it is straightened out."

Allen, with his seniority, seems insulated from layoffs, but younger workers like Sherry Hemming and Chuck Leach tend to blame Carter for some of Peoria's economic problems and their own concerns. Leach and Hemming are almost certain to hit the bricks if more layoffs come.

"Somebody's got to be better than Jimmy Carter," said Leach. "I'm not registered to vote, buy I'm thinking about it. Workers here seem divided -- some of them fear Reagan would bring on a war."

Hemming, working during vacation to build her savings, said, "The parties are all about the same to me. I'm not paying attention to the Republican convention, but neither one of them is worth the trouble."

A similar sense of detachment and helplessness echoes in remarks by white-collar men from Caterpillar, who declined to allow their names to be used.

I'm not saying I'm a Democrat or Republican, but I think we need a change. It is very depressing to me to see the auto and steel industries down. We need a turnaround and Jimmy Carter just isn't the answer," one said.

Another manager, a Democrat turned Republican after he left the production line, said "These labels don't mean much anymore, but I'm encouraged by Reagan. I like what I'm hearing from him. But, yes, it is a good question whether we can deal with four more years of another inexperienced president."

Whether it's Carter or Reagan or Anderson in November, whether the great television shows of the convention are watched or not, Peoria remains uneasy. No candidate appears to get the collective adrenalin pumping. r

"Further production curtailments may be necessary," said an official statement from Caterpillar last week, almost as an afterthought to a long financial press release. It's probably the worst news of the year.