Senate Majority Leader Robert C. Byrd (D-W. Va.) warned yesterday that President Carter's $50 tax-rebate plan is doomed to defeat unless a compromise is reached over the water projects the President is seeking to kill.

In an interview after he and other congressional leaders attended a breakfast meeting with Carter at the White House, Bryd said the tax rebate plan - a key element in the administration's economic stimulus package - "would lose" if voted on today in the Senate.

"Unless the water projects are resolved, in keeping with justifications (for their construction) made through the legislative process in the past, there is no possibility of winning," Bryd said. "If the water project matter is resolved, the tax rebate is potentially winnable."

Administration officials, meanwhile, give no hint in softening on the water projects dispute despite Bryd's warning on the tax rebate plan.

"We've indicated what our position is on the water pojects," said White House press secretary Jody Powell.

The jockeying between the senators - many of whom have pet water projects being threatened by the administration - the White House had all the earmarks of a game of political chicken leading up to the Senate vote on the rax rebate plan after the Senate returns from its Easter recess on April 5.

The administration currently is reviewing 30 water projects to determine whether funding for their construction should be continued. Citing economic, environmental and safety deficiencies, the President has said he believes all of the projects should be killed but that he will await the outcome of the review process before making a final decision.

Bryd's assessment of the status of the tax rebate plan was not unanimous among the Democratic leaders who attended the White House meeting. SeN. Alan Cranston (D-Calif) told "a tough fight" but "a winnable fight."

In other developments yesterday, the President named Alan K. Campbell, 53, the dean of the Lyndon B. Johnson School of Public Affairs at the University of Texax, to be the next chairman of the Civil Service Commission.

Campbell has been a consultant to the Advisory Commission on Intergovernmenttal Relations since 1969 and has served on various other government commissions.

It was also learned that Cater will deliver his second foreign policy address, and his first on Latin America, April14 at the Organization of American States headquarters here.