Jimmy Carter spent today doing the kind of things presidents have always done when they are trying to get reelected.

He rode in his big, black presidential limousine down the main street of a small town, where thousands of people from miles around turned out to share in the rare excitement of the day.

He stood for more than half an hour in the back yard of a wealthy supporter, the color drained from his face by the withering heat, to shake hands with people who had paid $500 to be there.

He flew in his Marine Corps helicopter to a stretch of parched farmland near here where he could express his sympathy for all the farmers and ranchers of the Southwest and Midwest who are suffering from the heat that is wilting much of the country.

But mostly, he raised money for his reelection campaign this fall against an earlier fund-raising appearance in Robards, Ky.

Democratic National Committee officials estimated that the three appearances would raise more than $800,000, with the money to be shared by the DNC and the Democratic state parties of Kentucky and Texas.

But the real beneficiary will be the Carter Reelection Campaign Committee, which has undertaken a belated attempt to close a fund-raising gap between it and the Republicans.

Under an amendment to the federal election law, state parties will be able to spend almost unlimited amounts this year for presidential candidates. Carter campaign officials fear the well-heeled GOP may be able to outspend the Democrats by millions of dollars, giving Reagan a distinct advantage.

To close that gap, the president was back on the road today, stopping in Henderson and Robards, Ky., before coming to Dallas.

Arriving in Kentucky early this afternoon, Carter sloughed off questions about his brother Billy's role as an agent of the Libyan government. "I don't want to comment about that," he said when asked about the call by Sen. Bob Dole (R-Kan.) for a congressional investigation of Billy Carter's activities.

The president rode in a 10-minute parade down the main street of Henderson, where thousands of people turned out to see him. The only sign of hostility was people protesting plans to locate three synthetic fuel plants in the agricultural area.

From Henderson, the president was taken to Robards and the home of Dale Sights, a wealthy banker who befriended the then-little-known Carter in 1975.

Speaking to about 500 people in a tent in Sights' back yard, Carter asserted that "not a single young American has lost his life in combat" during his administration, a claim he had dropped after eight U.S. servicemen were killed in the aborted rescue raid in Iran.

Fund-raising can be a tedious, sometimes painful labor, even for presidents, and it was today for Carter. He announced he would shake hands with everyone in Sights' yard, and so for more than 30 minutes he stood in the stifling heat as people filed past, and his staff grew increasingly nervous about his schedule.

His shirt drenched with perspiration, the president looked weak and uncomfortable in the heat, as first the paying guests, then state police who provided security for the event, and finally a number of women who had been cooking in Sights' kitchen joined the line.

One of Carter's aides, Robert Dunn, finally resorted to placing a flower pot at the end of the line to discourage other people from joining it.

The president arrived here late this afternoon and flew by helicopter to the farm of Olen Range to inspect damage to crops caused by the almost month-long heat wave here.

"My heart goes out to you," Carter told Range as he stood in a parched field holding a shriveled stalk of sorghum grain. He said he decided to stop at the farm to "dramatize to the rest of the nation" the plight of farmers who, according to state agricultural commissioner Reagan Brown, have lost $1.5 billion in crops in Texas alone because of the heat were.

Tonight in Dallas, the president amended his earlier remarks about combat deaths to include the losses suffered in Iran.

"With the exception of eight brave Americans who gave their lives in an accident in the desert of Iran, we have not had a single American life lost in combat since I have been president," he said.