In 1970, Congress passed legislation called the Racketeer-Influenced and Corrupt Organizations (RICO) Act that was designed explicitly to break organized crime groups. The special legislation was needed because organized crime groups were skilled at making more conventional prosecutions difficult. The Clinton administration used the RICO Act against anti-abortion activists. From a legal point of view, this was effective, but no one had ever envisioned the law being used this way when it was drafted. The government was taking the law to a place where its framers had never intended it to go.
Following 9/11, Congress passed a range of anti-terrorism laws that included the PATRIOT Act. The purpose of this was to stop al Qaeda, an organization that had killed thousands of people and was thought to be capable of plotting a nuclear attack. Under the same laws, the Bush administration has been monitoring a range of American left-wing groups -- some of which well might have committed acts of violence, but none of which come close to posing the same level of threat as al Qaeda. In some technical sense, using anti-terrorism laws against animal-rights activists might be legitimate, but the framers of the law did not envision this extension.
What we are describing here is neither a Democratic nor a Republican disease. It is a problem of governments. They are not particularly trustworthy in the way they use laws or programs. More precisely, an extraordinary act is passed to give the government the powers to fight an extraordinary enemy -- in these examples, the Mafia or al Qaeda. But governments will tend to extend this authority and apply it to ordinary events. How long, then, before the justification for tracking telephone calls is extended to finding child molesters, deadbeat dads and stolen car rings