In the 1930s, the Federales sent Al Capone to prison for tax evasion because they couldn't prove his guilt for the other stuff. This eventually led to Tom Cruise's line in the film The Firm, "It's not sexy, but it has teeth!"
Some people see the convictions of Martha Stewart and Barry Bonds, and now the prosecution of Denny Hastert, as a continuation of the Al Capone approach. I do not. With Capone, tax evasion was a crime in its own right. In the modern instances, the "crime" has only been outlawed because it is assumed to coincide with another crime. This is how civil asset forfeiture is justified. If you have $10,000 in your car, you must be a criminal, so we don't need to figure out what, exactly, you did that was illegal.
Martha Stewart did not go to prison for insider trading, she was convicting of not aiding the investigation of her alleged insider trading. Barry Bonds was not convicted of using illegal substances, he was convicted of not aiding the investigation of his alleged use of illegal substances. And Denny Hastert is not being prosecuted for sexually assaulting a minor, he is being prosecuted for accessing his own money in a way that was designed to avoid notice.
We don't defend these people because they are public villains. The government is using their villainy to normalize the prosecution of those who do not kowtow to government bullying. We're all less free because we can't stomach defending Martha Stewart, Barry Bonds, or Denny Hastert.