As the first orange tinges of morning crept over the gray cornfields, Donald Shasteen, age 49, pulled his floppy red-and-white hat tight over his bald head, looked down at his Adidas running shoes, and said, "Watch out for trucks."

Then he and two companions began, one foot after the other, running down U.S. Highway 30, beside the Platt River, running for the U.S. Senate.

He moved out past the Ames Farmers Union co-op elevator, a politician on the fun. Running hard, running scared.

Never has a politician in this state - maybe the nation - run so hard or so far, 470 miles to be exact, across the state of Nebraska from Wyoming on the west to Iowa on the East.

It clears his mind, he says. "Solutions to problems come to me while I'm running. I put together speeches and press conferences in my head."

Shasteen has run through rain; he's run through the hottest heat of summer. A German shepherd chased him into his camper truck in Central City; somewhere between there and North Bend, a bull snake struck at his shins.

It takes a wild jump of imagination to explain how all of this will make him a U.S. senator. But Shasteen is a physical fitness freak, a true believer in salvation by jogging.

Besides, what does a politician do when he's umpteen points behind in the public opinion polls, and big money men yawn when he drops by. A gimmick is a gimmick. And after all, didn't Dick Clark and Lawton Chiles get to the Senate by walking across their home states?

"When I started out, I had to look in the mirror to get somebody to believe that I had any chance of winning," admits Shasteen, who until Jan. 1 was administrative assistant to Sen. Carl Curtis (R-Neb.).

Now Shasteen wants Curtis' seat. Even though he is the nominee of the Republican Party in the most Republican state in the union, his chances are slim. His opponent is Gov. J. James Exon, Nebraska's most popular political figure. By almost everyone's account, Exon is far ahead in the race - without jogging a step - and there are few signs that Shasteen with his "I'm running for the U.S. Senate" T-shirt is closing in on him.

He isn't even sure that the run has gotten him much more than a folder full of newspaper clipping from almost every weekly newspaper along U.S. 30, although he thinks people are beginning to notice him.

"My strong suit is my knowledge of the job," says Shasteen, who has worked 10 years in Curtis' office. "But how do I get that across when nobody even knows my name and I can't afford to buy television time."

After seven months in the race, he has been able to raise a campaign war chest of only $35,000 - a pitiful sum even in a sparsely populated state like Nebraska. "In the long run," he says philosophically, "it at least shows that I have the grit and determination to do something no one else has ever done."

For the record, the Democrats aren't impressed - either in the long run or the short run. "I wouldn't vote for him if he could walk across water," one said the other day.

"I don't see that this is making much impact in dislocating the strong sentiment for Gov. Exon," says state Democratic Chairman Richard White. "It looks like little more than a desperate attempt to get attention."

"I'm not sure it is a positive thing to do," White continued. "It's something to keep him busy, but I don't see it doing him much good yet. Maybe he wants to open a health food store when it's all over. It could help him then."

Shasteen became a born-again jogger quite by accident about five years ago. At that time his oldest son was trying to get into the Air Force Academy, and needed to get in shape.

So father and son started jogging around their neighborhood in suburban Maryland. The son was admitted to the academy and promptly stopped running. The father, then a portly 210 pounds, kept going.

Within a couple of years, his weight dropped down to 155 pounds, and he became such a fitness freak that he was spending up to 90 minutes working out each day before he headed for Capitol Hill.

When Exon's popularity scared off better-known Republican opponents, Shasteen moved back to Nebraska where he had been a reporter for the Omaha World-Herald.

The state already has one Democratic senator, Edward Zorinsky, a former Republican who switched parties. And if Exon wins, it would be the first time in Nebraska's 110 years of statehood that this bastion of Republicanism, formed in the passions of the Civil War, would have two Democrats in the U.S. Senate.

Initially, Shasteen thought of the idea of running across the state, but rejected it. He thought jogging was too radical a gimmick for conservative Nebraska. Maybe Wisconsin, with its tradition of LaFollette progressivism, could stomach Sen. William Proxmire jogging all over the place. But Shasteen didn't think the Cornhusker State was ready for it.

But then he had breakfast with one of the state GOP's big money men. "He told me," Shasteen recalls, "unless you do something to get some name recognition. I'm not going to give you a dime. You're going to lose unless you do something dramatic like Dick Clark did in Iowa in 1972."

So Shasteen put on his Adidas running shoes. He began his jog on the Wyoming border in June and has been at it ever since. He runs from seven to 13 miles a day in the early morning hours, finishing about 8:30 a.m. when he returns to normal campaigning. He picks up the next day where he left off.

Thursday, for example, he started off at the city limits of Ames, an unincorporated farming hamlet on the eastern edge of the state, with two supporters at 7 a.m. Five teen-agers joined him about halfway through the eight-mile course. By that time, a light drizzle was falling. And when a car or truck passed by it would kick or spray on the joggers. But Shasteen didn't slow his steady pace.

His aide, 280-pound Dick Hanson, followed in a camper truck with Shasteen stickers pasted all over it. He watched as Shasteen waved at each passing vehicle, zig-zagging from the pavement to the weed-covered shoulder. When one truck came perilously close to them, Hanson said, "That must be a Democrat."

Shasteen ended the cross-state run yesterday at Blair, Neb., on the Iowa border.A crowd of supporters finished the last half-mile with him.

Shasteen says he is going to continue challenging Exon to a foot race. "I think all candidates should do this," he declares. "Every politician should be made to sweat once in a while."