President Carter has decided to replace the two incumbent, Republican U.S. attorneys in Virginia and soon will nominate Demoicrats for their jobs, a top White House official said yesterday.

In addition, the White House has sought to assure Virginia Democratic gubernatorial nominee Henry E. Howell that he and other Virginia Democrats will have a say in who the President selects for four proposed federal judgeships in the state, deputy White House appointments secretary Timothy Smith said in an interview.

The decision to replace the two prosecutors came after Howell, a long-time friend of the President, voiced displeasure last week at an earlier White House action allowing Virginia's Independence Sen. Harry F. Byrd Jr. to submit judicial nominations to Carter for the state.

Neither Smith nor Howell would say yesterday who the likely candidates are for any of the positions, although Smith said "some pretty strong consensus candidates have emerged" for the U.S. attorneys' jobs.

"That's the first I've heard about it," said U.S. Attorney William B. Cummings of Vienna, who has held the $43,500 a-year job as chief Prosecutor for the eastern porton of the state since March 1975. "I don't say I'm surprised," he said. Cummings said he has heard rumors about losing his job since Carter took office in January.

Howell said yesterday he was delighted that the White House was beginning to move on replacement for Cummings, a nominee of Republican Sen. William L. Scott and western district prosecutor Paul R. Thompson of Roanoke, another Scott nominee. Thompson's position, which typically handles fewer cases than Cummings', currently pays $39,500 a year, a Justice Department spokesman said.

The salaries make both positions more lucrative than most state prosecutors' jobs in Virginia and thus among the more choice political plums in the state. Appointees ate subject to confirmation by the Senate and serve at the President's pleasure.

Despite his delight with Smith's announcement, Howell said yesterday he hoped the White House didn't move too rapidly on the appointments since he had not had time to consult with members of the General Assembly leadership on who should be named to the positions.

Howell also disclosed that he urged the White House to name Roanoke City Sheriff Paul Puckett to replace Republican William A. Quick as U.S. Marshall for western Virginia.Puckett was a Howell supporter in Howell's recent upset gubernatorial primary victory over Andrew P. Miller. Republican Issac G. Hylton of Norfolk is serving as marshall in the eastern half of the state, but Howell said he has not recommended a replacement for him.

Smith said one reason the White House now has decided to proceed with naming replacements for the prosecutors was the failure of two commissions Byrd has formed on judicial nominations to consider replacing the men. However, Cummings said yesterday he had been told the Byrd commission in the eastern district didn't act on his job "because they didn't believe the position was vacant."

In addition to Howell, Smith said Virginia's four Democratic congressmen, Sen. Byrd, and "leading academies" in the state's law schools would be welcome to offer nominations to Carter for any of the positions.