Some states provide legal protection for activity outside the work place. For example, California Labor Code Sections 1101 and 1102 prohibit employers from interfering with their employees' political activities. If you blog about something political in California, these laws may protect you.
In addition, we believe that two relatively new provisions of the California Labor Code that address "lawful conduct during nonworking hours away from the employer's premises" could be used to protect employee bloggers. On their face, the provisions prohibit employers from disciplining employees for anything that goes on outside of work so long as it's legal (including, presumably, blogging). See Cal. Labor Code §§ 96(k); 98.6. So far, however, courts have not construed the law so broadly. Some courts and the Attorney General have ruled that these provisions merely help enforce existing rights -- such as the right to privacy -- rather than give employees any additional rights. See Grinzi v. San Diego Hospice Corp., 120 Cal.App.4th 72, 86-88 (2004); Barbee v. Household Automotive Finance Corp., 113 Cal.App.4th 525, 534-36 (2003). While the issue is not settled, these rulings suggest that Section 96(k) and 98.6 apply only if, in the course of terminating you, the employer violated some right that you have under other laws (for example, by interfering with your political activities, or libeling you).
Some states other than California (Colorado, New York, and North Dakota) have also enacted statutes that prohibit employers from terminating employees for engaging in lawful activities outside of work, including political activities. But these laws also have broader exceptions than California's and some do not apply to disciplinary actions other than firing. The District of Columbia, Connecticut, and some cities (for example, Seattle, Lansing, and Madison) also prohibit discrimination on the basis of political or expressive activity. For links to these laws and a discussion of the protections they provide, see the Workplace Fairness FAQ on Retaliation for Political Activity.