Richmond Circuit Court Judge James E. Sheffield emerged yesterday as the Carter administration's most likely nominee to fill a federal judgeship vacancy in Virginia, a move that would lake Sheffield the state's first black federal judge since Reconstruction.

The nomination is likely to set off a new confrontation between the administration and Sen. Harry F. Byrd Jr. (Ind.-Va.), who has not endorsed Sheffield for the post.

Sheffield's name was first mentioned yesterday at a Richmond press conference held by Lt. Gov. Charles Robb, who said he considered Sheffield and State Sen. J. Harry Michael (D-Charlottesville) the leading candidates for the state's two remaining judgeship vacancies.

An administration source in Washington later confirmed that Sheffield and Michael, whose name first surfaced in connection with the job earlier this month, were the leading contenders for the two posts. The source cautioned that neither would be nominated until the Justice Department conducted background checks that could take several weeks.

The two posts are among four newly established judgeships that have sparked months of controversy between Byrd, who submitted a list of 10 white males as candidates for the jobs last fall, and Carter, who wanted a list containing at least some blacks and women.

Last December, Carter threatened to leave all four positions vacant unless Byrd altered this list. Since then, the president has nominated two of Byrd's candidates, Richard L. Williams and James P. Jones, whose nominations are now before the Senate Judiciary Committee.

Byrd was unavailable for comment yesterday, but Jack F. Davis, his administarative assistant, hinted Byrd would not accept Sheffield's nomination.

"The senator's position is clear" said Davis in a prepared statement. "He stands firmly behind the recommendations of the judicial advisory commissions" (which chose the 10 white male nominees).

Davis noted it was Carter who had suggested the citizen commissions to make judgeship appointments less political. "Sen. Byrd can't conceive that a president would repudiate his own moral commitment, forcing a direct confrontation between the president and Sen. Byrd," said Davis.

Although Byrd is technically an independent, he still enjoys Senate seniority privileges granted to members of the Democratic majority. Those privileges include veto power over nominations such as judgeships in Virginia.

Neither the Justice Department nor the White House would comment on the likelihood of Sheffield's nomination yesterday, nor on Byrd's position.

Lt. Gov. Robb said he had been told by Justice officials that the nominations would be made within the next few weeks.

"I think it's pretty clear that President Carter is committed to nominating a black for one of the positions," he said.

Robb, a likely candidate for the Democratic gubernatorial nomination in 1981, has been under fire from women and blacks ever since he expressed sympathy with Byrd's position earlier this year.

He recently has been reported as playing a behind-the-scenes role in trying to mediate the dispute between Byrd and Carter.

Sheffield, who in 1974 became Virginia's first black circuit court judge since Reconstruction, was out of town and unavailable for comment yesterday.