Although he works just six miles from the White House and its Democratic occupant, Republican William B. Cummings seems to have a secure lease on his job as the federal prosecutor for eastern Virginia.

The Carter administration has so far appointed 67 new U.S. attorneys throughout the country and only one of them is reportedly a Republican. But Cummings is one of the 27 prosecutors appointed by the Nixon and Ford administrations who are still holding on to their jobs despite the Democratic takeover of the White House.

Moreover, a Justice Department spokesman says Cummings will probably keep his $46,600-a-year job "for a long while," possibly until his four-year term expires in March, 1979. The reason, according to the Justice spokesman, is that "there are no strong contenders" to replace Cummings.

Virginia Democratic leaders dispute that contention and say they have plenty of qualified candidates for the job. But the patronage system under which U.S. attorneys are normally appointed - is asked in Virginia since there is no Democratic senator or governor to put in a good word for any prospective applicants for Cummings' post.

As Lynn Johnson, a member of state Democratic committee trying to improve relations with the White House, explained, it didn't help when the state's Democrats were unable to deliver the votes for President Carter in the 1976 election. Virginia was the only Southern state Jimmy Carter did not carry.

"We have many good Democrats who are qualified and can fill the position," said Virginia State Democratic chairman Joseph T. Fitzpatrick.

"I have no gauge on whether he's effective," Fitzpatrick said of Cummings' performance. But he added, "a Democrat could do a better job than Cummings is doing."

An aide to Sen. Harry F. Byrd Jr. (I-Va.) said "there are no changes in sight" in Cumming's status and that the first move to replace Cummings with a Democratic appointee would come from the Carter administration.

In the past, U.S. attorneys resigned when new administrations took office. Senators of the same party as the President looked forward to nominating their choices for the position. But the Carter administration says it is selecting candidates on merit, not political leaning.

Nonetheless, U.S. attorney in Detroit, Republican Philip Van Dam, was fired by President Carter last May when he refused to resign. Attorney General Griffin Bell said at the time that Van Dam was tired mainly because "we had an election last year and the Democrats won. You can use your imagination after that."

In September, Jonathan L. Goldstein, a U.S. attorney in New Jersey known for his prosecution of local politicians and organized crime, resigned under White House pressure.

In his forced letter of resignation. Goldstein said Carter and Bell "have complishments on behalf of the United States must give way to the dictates of politics."

Under Goldstein, the U.S. attorney's office prosecuted two secretaries of state, Republican and Democratic state treasurers; the chairmen of both the Republican and Democratic state committees; 12 mayors, two congressmen; the president of the state senate and the speaker of the state assembly.

Cummings has not had a string of prosecutions like Goldstein, one of the few criticisms leveled at Cummings by other lawyers and his former associates.

Cummings said he has at least one investigative grand jury meeting all the time to dig up corruption. "But you can't invent corruption," Cummings said.

Still, in the first two years of his four-year term, Cummings has achieved a reputation for prosecuting more white-collar criminals and fraud cases than his predecessor, Brian P. Gettings, who resigned to enter private law practice.

Cummings said he has three of his 22 assistants zeroing in on fraud cases in his distric's offices in Richmond, Norfolk and Alexandria.

Last May a federal judge in Alexandria dismissed an indictment charging fraud against Litton Industries which was accused of cheating the Navy out of $37 million.

Judge Albert V. Bryan Jr. dismissed that case because he said U.S. prosecutors had interfered with the grand jury investigating the company and jury investigating the company and threatened Litton with the indictment if the company did not agree to variout proposals.

In a scathing attack on the prosecutors, Bryan said they had abused their prosecutorial discretion.

"What is reprehensible here is the threat to use as well as the actual use of the grand jury as a bargaining tool . . ." Bryan said.

"This is a further example of the cynical view that has been taken of the grand jury in this case, namely as a mere echo of the office of the U.S. attorney," Bryan said.

"I don't think the situation is bad," Cummings said, defending his assistants. "It looks bad."

Just last month, three ex-Navy officers were acquitted of charges that they had conspired to induce the Iranian Navy to sign a training contract with a private company in which they had an interest.

"We had a well-tried case," Cummings said. The jury just decided against the government's case, Cummings said.

Cummings, 38, was a Fairfax County attorney active in Northern Virginia politics before he was nominated in 1974 for the prosecutor's job by Sen. William L. Scott (R-Va.) Cummings was selected after the first nominee for the job withdrew his name because of vocal objections from local attorneys and a second Scott nominee withdrew for personal reasons.

Some attorneys have chiticized Cummings because his background is mostly in civil, not criminal law, a fact Cummings acknowledges. He also personally prosecutes very few cases and is know more for being an administrator.

Cummings cites as his successes:

Thomas F. Regan - was convicted of accepting bribes and engaging in fraud while head of the Small Business Administration office in Richmond.

A former Richmond General District Court deputy clerk pleaded guilty to a conspiracy charge in connection with the illegal sales of guns.

A Reston doctor was convicted last May in Alexandria of Medicaid and Medicare fraud.

Wesley T. Butler, developer of the Captain's Cove vacation home project, was convicted last June of making false statements to government investigators.