Morris Dees, Jimmy Carter's chief fund-raiser in 1976, abandoned the president's reelection campaign yesterday to become national finance director for Sen. Edward M. Kennedy.

The defection stunned political operatives at the White House, where Dees had been counted on as a key figure in Carter's reelection effort. During the past summer, Dees, an expert on mass mailing, sent out letters under the name of Lillian Carter, the president's mother, to raise funds for the 1980 campaign.

Although Kennedy has not formally announced his candidacy, he has been contacting political figures throughout the nation about his intentions. A Kennedy spokesman said the Massachusetts senator will announce Monday the formation of an "exploratory" committee, the final step before a formal declaration of candidacy.

"This will be a real blow to the president's campaign," said a Carter campaign source who also worked on his 1976 campaign. "Morris is the guy who kept the campaign afloat during the early primaries when we needed money."

After informing the White House of his decision, Dees told The Los Angeles Times, "I can still have great respect for Jimmy Carter, but I'm philosophically more comfortable with Sen. Kennedy."

Dees, 42, a wealthy Montgomery, Ala., attorney and business executive, said he still considers "virtually all of the White House Staff to be my friends." But he added, "we're going to elect Sen. Kennedy."

His initial plans, Dees said include scheduling fund-raising dinner in Los Angeles, New York and four or five other cities between Dec. 10 and dec. 16. The goal will be to raise "a quick couple of million dollars" before Jan. 1, he said.

"Then we'll organize finance committees in every state and begin committees in every state and begin grassroots solicitation. We'll use direct mail extensively and I think it will work well for Sen. Kennedy. We hope to send out the first million pieces of mail withing the next six weeks."

Dees said he would not have supported Carter in 1976 if Kennedy had run and would not have written the Lillian Carter letter had he known Kennedy planned to run in 1980.

Kennedy's political operatives had been discussing the campaign financing post with Dees for the past month. Dees said Paul Kirk, a Washington and Boston attorney who advises Kennedy, offered him the post and he finally accepted 10 days ago.

He said he informed Hamilton Jordan, Carter's chief of staff, of the decision during a White House meeting yesterday so that Jordan "would not read about it in the newspaper."