I've written in defense of negative campaigning before, and I've floated that editorial on the blog a couple times the last two years. Here is another important point to consider.
Some people do not like to go negative and will refrain from doing so. An incumbent running for re-election may want to focus on his record rather than criticize his opponent, and someone running for an open seat may want to only talk about his qualifications for the position and why he should be chosen for that position. In a primary election where two very similar candidates are seeking an open seat, a lack of negative campaigning would not be unusual.
This is not something I agree with, as voters should hear all of the information about the candidates, both positive and negative, in order to make an informed choice. But there are some candidates who have an obligation to go negative against their opponents, and those candidates are the ones challenging an incumbent. After all, the very fact that the challenger is on the ballot is a statement to voters that the incumbent is unqualified to continue serving in office. If the incumbent was doing a fine job, then why bother challenging him?
Simply put, the voters deserve an explanation as to why they should remove the incumbent from office. What has the incumbent done wrong? Has the incumbent abused his authority? Has the incumbent mismanaged his office? Does the incumbent advance policies that are ineffective, counterproductive or destructive? In a primary election, does the incumbent not represent the majority of the party?
When someone is running for office against an incumbent, the burden of proof is on the challenger as to why the incumbent should be removed from office. If the challenger cannot or will not show voters why the incumbent should be removed, why would anyone vote for that challenger? Why not simply keep the incumbent in office?
If a challenger does not articulate why the incumbent should be removed, then the message sent to voters is not that we need to go in a new direction, but that the challenger is seeking office for his own personal ambition. Candidates who are seeking to remove an incumbent from office purely for personal ambition instead of the desire to make a substantive change (especially in a primary) do not deserve to be elected.