From Vice News:

“If there were a conspiracy, then the order to reach out to Roger Stone might be an overt act that would make the conspiracy complete,” said Paul Rosenzweig, who was a member of Ken Starr’s investigation into former President Bill Clinton.

But there’s also a special type of conspiracy charge — one that doesn’t need to be based on an underlying crime. The dramatically titled “conspiracy to defraud the United States” is particularly well-suited to Mueller’s purposes.

Just last Sunday, Rep. Adam Schiff (D-Calif.), the top Democrat on the House Intelligence Committee, told ABC’s George Stephanopoulos that a broad, sweeping “collusion indictment” may be Mueller’s final charge.

“If there is a conspiracy to defraud the United States, a collusion indictment, it would be the last indictment Bob Mueller would seek, not the first,” Schiff said.

The charge is also known as a “Klein conspiracy,” after an otherwise obscure 1957 tax case.

In that instance, a group of whiskey merchants including a guy named Hyman Klein was charged with both tax evasion and organizing a complex scheme to stop the Treasury Department from collecting taxes. While they beat the tax evasion charges, they were found guilty of the conspiracy itself.

A “Klein conspiracy” involves an agreement to obstruct a function of the U.S. government, such as collecting taxes. In this case, the conspiracy may involve interfering with an election.

“I think a Klein conspiracy is highly likely to be the direction that Mueller is going,” said Rosenzweig. “Mueller would need to demonstrate that there was an agreement to undertake an unlawful objective: to interfere with an election in the United States, and that at least one overt act was done in furtherance of that conspiracy.”