A lawyer for the Bush administration ran into aggressive questioning by justices who wondered why the mere concealment of cash in a car headed for Mexico meets the standard for an international money laundering charge, which carries a maximum penalty of 20 years in prison.
"All you have here" is transporting cash and concealing it, complained Justice Antonin Scalia.
A person who prevents detection of illegal proceeds "is exactly what Congress had in mind" when it passed the law, replied Lisa Schertler of the Justice Department solicitor general's office.
It is the second such case the court has heard this term.
Four months ago, the justices expressed other concerns with the government's use of the money laundering law in a gambling case. That case is still pending.
When Congress cracked down on the crime of money laundering in 1986, it had two problems in mind: drug cartels and organized crime. The law makes it a crime to transport money across the border when it is designed to conceal the source, ownership or control of the funds.