George Bush's high-riding GOP presidential campaign was ambushed today in Arkansas.

In balloting that selected the first delegates of the 1980 presidential campaign, Bush won only one, the last of 12 elected at four congressional district caucuses in the state.

Ronald Reagan, whose support was thought to be sagging after Bush defeated him in the Iowa precinct caucuses, won six delegates. Sen. Howard H. Baker Jr., whose campaign has been desperate for any good news, finished a surprisingly strong second with four. Baker's aides also claimed the support of a fifth delegate technically elected and uncommitted.

John B. Connally, who spent considerable time and money here, was shut out without a single delegate and was the biggest loser of the day. This was the second straight poor showing by Connally, a weak fourth-place finisher in Iowa, and raised serious questions about his claims of widespread support in the South.

The results are a clear blow to the momentum Bush built after his upset victory in Iowa Jan. 22. Aides had predicted that Bush, who made a major effort here, would win at least five delegates. The results are also expected to buoy sagging spirits among the Reagan forces, which had claimed the Arkansas delegate election process was stacked against them.

Bush forces blamed the defeat on political wheeling and dealing between Reagan and Baker. "There was a very clear pattern of trading votes to stop George Bush," said state campaign coordinator Shirley Green. "It appears to me that the other campaigns view George as a threat and decided to stop him in Arkansas."

There was ample evidence of vote swapping. Reagan, for example, didn't challenge Baker in his stronghold, the second congressional district, which includes Little Rock. Baker forces in turn didn't challenge Reagan delegates in four of the six delegate contests in two other congressional districts. The two Baker delegates in these districts won by almost the same margins as did the Reagan ones.

But Paul Manafort, Reagan's convention state coordinator, argued that the results showed that Bush, a virtual political unknown a few months ago, had not developed a strong political base in the state.

"Maybe he isn't as hot as everyone thought," he said.

Although the Arkansas vote attracted little national attention and involved only a handful of Republican loyalists, it marked the first selection of actual delegates to the 1980 national convention. The highly publicized Iowa precinct caucuses were only the first step in a four-tier process leading to selection of delegates.

The balloting was pretty much a closed-shop affair, the closest thing to a smoke-filled room that remains in American politics. Only 181 party members took part in the small caucuses held at four locations. Twelve delegates, the first picked by either party in the nation, were elected. Seven more Republican delegates are to be chosen at a state convention Feb. 16.

Democrats have set a separate May 27 Arkansas primary.

Doyle Webb, a young law student at the University of Arkansas, became the first delegate elected in early morning voting in Little Rock. A Baker supporter, he won by a razor-thin 15 to 14.

The second delegate picked was Bill Kelly, a long-time GOP kingpin in the Little Rock area and the Pulaski County Republican chairman. He was elected as an uncommitted delegate, but Baker forces claimed him as a supporter.

The selection process appeared to favor Bush, a former GOP national chairman, CIA director and ambassador. His campaign has built its reputation on its ability to organize relatively small numbers of party members.

There was no better example of what happened statewide than the third congressional district caucus. The district was supposed to be Bush's biggest stronghold. Located in the northwest quarter of the state, it includes the old gambling and resort center of Hot Springs; Fayetteville, home of the University of Arkansas, and Fort Smith, on the Oklahoma border.

For most of the century, it has been the only congressional district in the state with real two-party contests.

Bush had the support here of the district's popular Republican congressman, John Paul Hammerschmidt, and he hoped to sweep at least two and possibly all three of the district's delegates. In addition, Bush spoke before a packed, $20-a-plate fund-raising dinner for the county Republican organization Friday night, and made a personal appeal to the caucus delegates today for their support.

But he was clearly outmanned here. A backstage political maneuver between Baker and Reagan forces caught Bush supporters by surprise. And Bush didn't get a single delegate.

In exchange for Reagan support in electing Baker state chairman Len Blaylock as a delegate, Baker forces helped elect two Reagan delegates, including Reagan's state chairman, Sharon Shipley.

"The consensus here seems to be to gang up to stop Bush," Blaylock, the 1972 Republican gubernatorial candidate, said later.

"Oh, man, the perils of winning in Iowa," Bush field worker Linda Allison complained. "I couldn't work in politics for 25 years without recognizing a deal."