LEVEN YEARS after leaving office, this country's oldest living former president has decided that the taxpayers need no longer spend $3 million a year to provide him with armed guards 24 hours a day, 365 days a year, at home and abroad. Richard Nixon has voluntarily given up Secret Service protection and announced his intention to hire private security guards for himself, his wife and their home in suburban New Jersey. It is a commendable step.

The responsibilities of the Secret Service have increased enormously in the past 20 years or so. Initially assigned to guard the president, vice president, their immediate families and assorted visiting heads of state, the service now provides protection for former presidents, their spouses and minor children, widows of former presidents and, every four years, presidential primary candidates and the nominees of major political parties. You can't work your way through the halls of a Senate office building in an election year without encountering dozens of serious young men and women wearing lapel pins and buttons in their ears.

In addition to the Secret Service guards, most executive departments and some elected officials have their own security forces providing protection during public appearances. And just this week, the justices of the Supreme Court, citing anonymous threats received by some of them, asked Congress to provide them with escort service between their homes and the court.

No one begrudges protection for high government officials. In this age of increasing terrorism, vigilance is necessary. But providing round-the- clock security for former officials, their families and widows has a lower priority, because the threat to the national security in their cases is minimal. Mr. Nixon, in giving up his Secret Service privilege, acknowledges this fact and sets a good example.