The political tides are not running here the way one might expect. If other combination farm-industrial districts nationwide react to the sliding economy the way this one appears to be doing, President Reagan and the Republicans have nothing to worry about.

"The president can't do anything and the Congress neither. I don't really believe there is a solution," said Louis Null, owner of three semi-trailer trucks, over breakfast in Grandma's Kitchen in Plymouth, 20 miles south of here. "I vote so if I don't like what happens I can gripe."

A lifelong Republican, Null said he sees no reason to switch parties just because unemployment here is swelling, mortgage foreclosures are swamping the courthouse and the factories are running at half to two-thirds capacity. He will vote again for Rep. John Patrick Hiler (R-Ind.), a baby-faced, hard-core freshman conservative so devoted to Reaganomics that he has criticized the president for going soft on his own programs.

Hiler's polls show him leading his Democratic challenger, veteran state representative Richard C. Bodine of Mishawaka, by nearly 20 points. Democratic leaders are glum even though Bodine has closed somewhat.

"It's do-able," insisted Bodine's media coordinator, Frank Yurasek. "The same thing that brought Hiler in can take him out."

Hiler, 29, the marketing director of his family's metal foundry, won national notice in 1980 by knocking off House Majority Whip John Brademas, a Democrat who was first elected to Congress when Hiler was 5.

Hiler ran as an ardent supporter of Reaganomics, successfully wooing the independent and farm votes that Brademas took for granted and won with 55 percent of the vote.

In Congress, Hiler has been a throwback to the ancient days of freshman obscurity, offering little legislation but seconding his elders as a rock-solid Reaganite. House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) called him and 51 other first-term Republicans "Reagan's Robots," predicting they would change their wiring once Reaganomics short-circuited their districts.

Times are tough now here, with unemployment 11 percent statewide and 15 percent in the auto parts factories around South Bend. Rural counties record three or four mortgage foreclosures a week. It used to be one every four months.

But rather than moderating his pitch, Hiler is running on the slogan: "He's doing the job he was hired to do."

Echoing Reagan, Hiler still blames 22 years of Brademas and company for the bad economic news, saying that under Reagan things are starting to turn around.

"I agree totally," said Jeff Warstler of Goshen after shaking hands with Hiler in the ear-shattering assembly rooms of Smoker Craft boat company in New Paris. Warstler echoed others in praising Hiler for "keeping us informed."

Using his incumbency, Hiler has papered his 250,000 voters with newsletters and pamphlets, getting many printed verbatim as news items in the local papers.

Hiler plans to spend about $400,000 this year, but Bodine's supporters say their candidate will be lucky to raise one-fourth of that. After 18 years in the state legislature, Bodine, 49, was a last-minute entry, emerging from a vicious five-way primary in May with a divided party, no organization and no money.

His base is in South Bend, where Mayor Roger O. Parent, a Democrat, says Hiler's unwillingness to hunt federal help for the district "was a big adjustment for us after Brademas."

Still hurting from the 1967 demise of Studebaker, South Bend will soon lose a Bendix plant to the Sun Belt. Hiler didn't try to stop that, saying it wasn't his place to interfere with the free flow of capital.

Parent sighed. "Our only chance in this race is to get people upset enough over this kind of thing. But I'm not sure they're upset enough."