Try this thought experiment: You decide to donate money to Mitt Romney. You want change in the Oval Office, so you engage in your democratic right to send a check.
Several days later, President Barack Obama, the most powerful man on the planet, singles you out by name. His campaign brands you a Romney donor, shames you for "betting against America," and accuses you of having a "less-than-reputable" record. The message from the man who controls the Justice Department (which can indict you), the SEC (which can fine you), and the IRS (which can audit you), is clear: You made a mistake donating that money.
Are you worried?
Richard Nixon's "enemies list" appalled the country for the simple reason that presidents hold a unique trust. Unlike senators or congressmen, presidents alone represent all Americans. Their powers—to jail, to fine, to bankrupt—are also so vast as to require restraint. Any president who targets a private citizen for his politics is de facto engaged in government intimidation and threats. This is why presidents since Nixon have carefully avoided the practice.