Was anything they printed actually untrue? Bias isn't enough for a libel case - the allegation must be untrue.
Also, as this is within the context of a political race, it seems likely that the mayoral candidate would be treated as a public figure. If that is the case, it is not enough to simply show untruth - the plaintiff would have to show either that the newspaper actually knew that the allegations it printed were untrue, or acted with reckless disregard for the truth.
The mayoral candidate would likely benefit from discussing this situation with a local lawyer, and formulating a strategy to deal with this information (which very well may not involve litigation).

