Don Foley, press secretary to Democratic presidential hopeful Rep. Richard A. Gephardt (D-Mo.) and the only senior campaign staffer who has been with him since the start of his political career, resigned yesterday.

"There was something lacking in the relationship between me and the {other senior staff of} the campaign," Foley, 39, said. "The chemistry wasn't there. I didn't think I had the complete confidence of {campaign manager William} Carrick and some of the others."

Foley said there were no disagreements over the direction of the campaign, and said part of his reason for leaving was personal. "I decided I didn't really want to work in the White House," he said.

Foley, who was a college student in St. Louis in 1969 when he first met Gephardt, managed Gephardt's first campaign for Congress in 1976. He has served as his congressional press secretary since then, except for a leave of absence he took in 1984 to serve as deputy press secretary for Walter Mondale's presidential campaign -- a job he sought so as to prepare himself for an expected presidential bid by Gephardt.

Foley said yesterday he also plans to leave the congressional office, and seek a job in government relations. Since February, when Gephardt declared for the presidency, Foley divided his time between the campaign and the congressional office.

Carrick, who joined Gephardt about a year ago from Sen. Edward M. Kennedy's (D-Mass.) staff, called Foley's resignation "a total surprise" and a "real loss for the campaign." He said he had not yet decided on a successor.

Sources in the campaign said Foley has been distracted during the past year by family considerations.

His departure comes at a time when the campaign has been experiencing money problems -- it was late in making payroll twice this month, though Carrick said yesterday that everyone has now been paid -- and an apparent slippage of support in Iowa, where Gephardt has campaigned more heavily than any other candidate.

A Des Moines Register poll published last weekend showed Gephardt in third place with 14 percent of the vote. Last May, in the first poll taken after Gary Hart dropped out of the race, Gephardt led all Democrats with 24 percent.

"I knew that any time I left would be awkward," Foley said. "On the other hand, by doing it now, there's still time to bring someone else in to do this."