Ronald Reagan's manager said today he would continue his low-profile campaign strategy despite a newly published poll showing that he has slipped, and is now within range of an upset by George Bush and Sen. Howard H. Baker Jr. of Tennessee in the Jan. 21 Iowa Republican caucuses.

The Iowa Poll of The Des Moines Register and Tribune said Reagan had 26 percent of the votes, compared with Baker's 18 and Bush's 17, in a sampling taken the first three days of this week among 267 Iowa Republicans.

The same poll showed President Carter has moved out to a 57-to-25 per cent lead over Sen Edward M. Kennedy (D-Mass.) among 287 Iowa democrats with California Gov. Edmund G. (Jerry) Brown Jr. receiving 4 percent of the votes.

Carter's move from a 40-to-40 tie with Kennedy in December and a 26-to-49 deficit last August confirmed a trend sensed by both camps. Carter backers, fearful of exaggerated expectations, immediately began minimizing its significance, while Kennedy eagerly embraced the underdog role.

But the Reagan skid was worse than anyone in his camp, or most neutral observers, had expected. It was viewed with skepticism not only by Reagan's managers but also by other GOP leaders.

The margin of error inherent in polling, especially with a small sample, means that Reagan's lead could be nonexistent or greater or smaller than indicated.

In August and December polling, Reagan had been holding steady with 48 and 50 percent of the vote, comfortably ahead of his active challengers.

The poll attributed much of the sharp decline to Reagan's refusal to participate in last Saturday's televised Republican candidate forum, where Bush, Baker and four other GOP hopefuls appeared. By 7-to-1, the Republicans interviewed said the absence of Reagan from the debate had hurt him.

But Reagan campaign manager John P. Sears, who made the decision to keep Reagan out of the debate, said in a telephone interview from Washington that "we have not even discussed" remedial steps. "We have to decide on the basis of what we see coming out of our organization efforts," Sears said, "and those signs are all very affirmative."

Reagan, who has made four appearances totaling 34 hours in the state, is scheduled to be back one last time Saturday for a three-city swing, ending with a statewide television speech from a Des Moines rally. No other appearances are being added.

Bush and Baker, who have campaigned far more extensively, said they saw the poll as a vindication of their efforts.

Baker who dipped from 23 percent in August to 11 percent in December, and who has come back to 18 percent, described himself as "absolutely delighted." The Senate minority leader was plagued with organization problems until December, but has scheduled himself for 12 days of campaigning here in January. Dick Redman, Baker's Iowa manager, said, "I think the people are getting to know Howard Baker in Iowa and those who do like him."

Bush has had the steadiest climb in the polls -- from 1 percent in August to 14 percent in December and 17 percent this week. The earliest starting of the candidates in terms of organization, he is considered the best best to upset Reagan, if the front-runner is, in fact, stumbling.

Bush told reported today that "the trend line is what is important." Referring to Reagan's decline, he said, "It looks like we have another Teddy Kennedy situation brewing here."

Fourth place in the GOP poll went to John B. Connally with 1o percent, almost unchanged from his August and December figures. Eddie Mahe Connally's campaign manager, said Connally was "pushing hard for third place," and professed to see more problems ahead for Bush than for Reagan.

"I absolutely refuse to believe that Reagan fell 24 percent in three weeks," he said. "We perceived some slippage after the debate, but not 24 percent -- I'm sorry, I can't buy that."

On the contrary, Mahe said, the revival of Baker and the slowing of Bush's climb are "not a good omen" for Bush."

The trailers in the Republican field are Sen. Bob Dole of Kansas and Rep. Philip M. Crane of Illinois, both with 6 percent, and Rep. John B. Anderson of Illinois, with 1 percent.

Baker, Crane and Bush were rated as winners in the debate, according to the poll, while Connally, Dole and Anderson were seen as losers.

On the Democratic side, according to the poll, Carter largely escaped criticism for his decision to bow out of the scheduled Jan. 7 debate with Kennedy and Brown. By almost 3 to 1, Democratic voters surveyed accepted Carter's explanation that his decision resulted from the crises in Iran and Afghanistan, rather than being based on political considerations, as Brown had charged.

The poll showed 2-to-1 support among Democrats for the grain embargo against the Soviet Union, which Kennedy has made the center of his attack on Carter in his campaign swing this week. Farmers polled were opposed to the embargo, but ther views were overridden by city residents.

In the face of Carter's continued climb, Bill Romjue, his state coordinator, conceded that, "I really can't say we're the underdog any more." But he added that the poll "is a dangerous thing for us because it makes out people apathetic."

Kennedy, campaigning in Iowa City, seemed to take the news in relaxed fashion, and said it was no surprise. Kennedy said Carter "clearly much get 50 percent or above" in the caucuses "or that would be a major setback."

His Iowa campaign spokesman, Steve Johnson, went even further, saying that if Carter does not win the caucuses by the polls' 2-to-1 majority, it would be a psychological blow to the president.

Neutral observers in both parties noted that a caucus fight depends as much on organization strength as on candidate popularity. Carter, Kennedy and Bush are credited with having built the most expensive organizations, with Reagan's an unknown quantity and Baker, Conally and the other Republicans believed to be lagging well behind.