With his presidential campaign suffering major organizational problems, Sen. Howard H. Baker Jr. (R-Tenn.) switched campaign managers late yesterday.

Wyatt Stewart, a veteran Republican fund raiser and organizer, replaces Don Sundquist, a Tennessee businessman who directed the Baker campaign since April.

The move comes two weeks after Baker was narrowly defeated in a straw ballot vote in Portland, Maine -- a vote he was so confident he would win that he took a plane load of reporters with him to witness the event.

The vote deeply embarrassed Baker because it reinforced what his critics have been saying for months: despite a good showing in national polls, the Senate minority leader's presidential hopes are in trouble because he hasn't put together a dependable campaign organization in a host of early primary states, including Iowa and New Hampshire.

Earlier this week Baker withdrew from the straw vote that will be taken today at a Florida Republican convention, claiming a schedule conflict. The state GOP chairman, distressed, called it reneging on a commitment.

Spokesmen for Baker last night denied that the switch in managers was brought on by problems in the campaign. They said Sundquist, 43, had asked to return to his family and business in Memphis.

"At the expense of both Don Sundquist skillfully put the Baker campaign together and made it function smoothly even while Sen. Baker was unable to be a full-time candidate," Sen. Richard Lugar (R-Ind.), Baker's campaign chariman, said in a statement. "He has been instrumental in setting the stage for a winning effort."

"Don certainly wasn't fired," said Stewart. "This has nothing to do with organization efforts. I'm of the opinion that Don has done a helluva job."

Stewart added that the move was not part of any major effort to restructure the campaign. "I have no intentions of any wholesale firings at this time," he said.

But the timing of the announcement, just two weeks after Baker formally declared his candidacy, raised questions. Stewart said he was first contacted about the job "a week or 10 days ago" or just after the Maine straw ballot, the latest in a series of political defeats for Baker.

Stewart, 40, is close to John Deardourff and Doug Bailey, political media experts the Baker camp has hired. Stewart has been finance director of the National Republican Congressional Committee since 1975 and has served as a consultant to the Senate Republican Campaign Committee and the Republican National Committee.

Sundquist will become campaign vice chairman, a part-time job, with responsibility for liaison with Republican governors and party leaders.