When Henry Dargan McMaster was nominated to be U.S. attorney for South Carolina 10 days ago, it was the first such nomination to be sent to the Senate by the Reagan administration since it took office in January.

McMaster was nominated partly because he'd been an aide to Sen. Strom Thurmond (R.-S.C.), chairman of the Senate Judiciary Committee, which passes on all nominees for U.S. attorney. Also, the incumbent U.S. attorney for South Carolina died this year while still in office, and South Carolina obviously needed a new U.S. attorney.

At least 30 other states also have vacancies for U.S. attorney that the Reagan administration has failed to fill, however. Of the 94 U.S. attorneys, 31 have resigned, two have died and 15 more have submitted their resignations.

This means that about half those still in office, who must prosecute a rising tide of federal cases, are either lame ducks or acting U.S. attorneys who lack the political clout of a presidential appointment.

President Reagan's Justice Department has been slow to act on more than the U.S. attorney nominations. The Reagan administration has yet to send a single name to the Senate for the nine vacant judgeships in the U.S. Court of Appeals, three alone in New York's busy Second Circuit. The administration also has yet to send a single name to the Senate for the 43 vacant judgeships in the already overworked federal district courts.

"It is all very well to prefer the appointment rather than the election of judges," Second Circuit Chief Judge Wilfrid Feinberg observed in a recent speech, "but we must admit that the system of electing judges does not allow vacancies to linger for two years or more as we now have in the federal judicial system."

Reagan's attorney general, Wiliam French Smith, is still without assistant attorney generals in three critical divisions of the Justice Department: the Tax Division, the Civil Division and the Civil Rights Division. One result is that at least four prosecutions of school desegregation suits are languishing in what one line attorney called "legal limbo." They are in Charleston, S.C.; Lima, Ohio; Yonkers, N.Y.; and Marshall, Tex.

"These suits are all in what lawyers like to call legal review," one civil rights lawyer said. "That fact is, no review is going on at all because the Civil Rights Division has nobody at the head to review them."

Some Justice Department officials attribute the slow rate of appointments of judges and U.S. attorneys to the slowness of the Republican majority in the Senate in suggesting names. Others say the process of approving names has dragged partly because so many Reagan loyalists at the White House and the Justice Department, including Smith and some of his advisers, have been slow to approve people they don't identify with conservative Republicanism.

The result, one Justice Department source said, is a lowering of morale throughout the judicial system. Said the source: "For any system to work, there has to be pressure from the top. Right now, there are not enough people at the top to make the system work. There are not enough decision-makers in place."

Smith promises that things are about to change, that the name of a nominee to head the Civil Rights Division has been sent to the White House, that at least 10 are being considered for the federal bench and that as many as 26 candidates being considered for the U.S. attorney vacancies.

"The thing that has frustrated me the most in this job so far is getting our vacancies filled," Smith said in a recent interview. "Trying to find people to fit these vacant slots is an extraordinary difficult job."

What happens in the federal judicial system when vacancies aren't quickly filled? Everything slows down.

Says on former Justice Department official: "Things get deferred, everything from prosecutions to appeals. The fires still get put out but no long-term investigations take place, no tough indictments come up and the caseloads in the courts start piling up."

Take New York's Second Circuit, the busiest federal court in the country. There are five district court vacancies. There are three vacancies on the 11-bench Second Circuit Court of Appeals, which, to keep up with its caseload, is relying on visiting judges and senior judges whose average age is 81.

The number of cases filed in the Second Circuit Court of Appeals rose from 1,739 in 1975 to 2,171 in 1980. For the first nine months of the current statistical year, the number of filings in the Second Circuit reached 2,271, an all-time record.

Demands are so severe on the district courts in the Eastern District of New York, which serves Long Island, that the circuit counsel recently declared a judicial emergency to allow the courts to speed up their criminal trials. The emergency expires July 1, however, unless Congress passes special legislation to extend it. Meanwhile, the Eastern District has had a vacancy for the last two and a half years.

"Perhaps what we need is a counterpart of the Speedy Trial Act [of 1971] for judgeships," Judge Feinberg said. "The time has certainly come to give serious thought to a means of assuring the prompt filling of vacancies in the federal courts."