Ginsburg revealed in a speech in South Africa last month that she and O'Connor were threatened a year ago by someone who called on the Internet for the immediate "patriotic" killing of the justices.
Security concerns among judges have been growing.
Conservative commentator Ann Coulter joked earlier this year that Justice John Paul Stevens should be poisoned. Over the past few months, O'Connor has complained that criticism, mainly by Republicans, has threatened judicial independence to deal with difficult issues like gay marriage.
Worry is not limited to the Supreme Court. Three quarters of the nation's 2,200 federal judges have asked for government-paid home security systems, Attorney General Alberto Gonzales said this week.
Web threat: Ginsburg said the Web threat was apparently prompted by legislation in Congress, filed by Republicans, that would bar judges from relying on foreign laws or court decisions.
"It is disquieting that they have attracted sizable support. And one not-so-small concern -- they fuel the irrational fringe," she said in a speech posted online by the court earlier this month and first reported yesterday by LegalTimes.com.
According to Ginsburg, someone in a Web site chat room wrote: "Okay commandoes, here is your first patriotic assignment .
Rep. Tom Feeney, R-Fla., a sponsor of one of the congressional proposals, wrote about the legislation on his Web site and in bold letters featured a quote from O'Connor predicting the Supreme Court would probably increasingly rely on foreign courts.
Ginsburg pointed out that the legislation was first proposed in 2004, an election year.
Justices, in some of their most hotly contested rulings, have looked overseas. Last year, for example, justices barred the executions of juvenile killers on a 5-4 vote. Justice Anthony M. Kennedy said then that "it is proper that we acknowledge the overwhelming weight of international opinion against the juvenile death penalty."
In an angry dissent to that decision, Justice Antonin Scalia said capital punishment policy should be set by states, not "the subjective views of five members of this court and like-minded foreigners."
Ginsburg said, "Critics in Congress and in the media misperceive how and why U.S. courts refer to foreign and international court decisions." She said those decisions are used for guidance only.
O'Connor said last week during a speech at Georgetown Law School that the justices have received threats. But the Ginsburg remarks at the Constitutional Court of South Africa provide unusual detail.
Ginsburg, who turned 73 yesterday, told the audience O'Connor "remains alive and well -- as for me, you can judge for yourself."