Whereas the conventional general election is vicious from the very start, primary contests usually maintain a faade of intra-party for a certain period of time. By the last week before the primary, that time is passed.
The first and most prominent example is Scott Walker's barrage of negative attacks against Mark Neumann. Walker, the frontrunner, perceives Neumann's chances are not as bad as the GOP establishment would like to believe, so he's running attack ads accusing Neumann of being a pork-barrel spender in conservative clothing.
Or look at the Republican primary for the 8th district Congressional seat. Despite a rather cordial contest between Reid Ribble, Roger Roth and Terri McCormick, the latter of the three lashed out at Roth, an Iraq war veteran, effectively accusing him of having a luxurious war experience. This was a couple weeks after Roth accused both of his opponents of supporting amnesty for illegal immigrants.
Then there's the exciting Democratic Senate primary in Milwaukee, between incumbent Sen. Jeff Plale and Milwaukee County Supervisor Chris Larson. The tone of that contest has never been especially friendly anytime somebody challenges an incumbent in a primary, he has to make a pretty strong case against him to win. However, it is only in the past few weeks that the campaigns have started to really dig deep into the dirt, with both campaigns using their opponent's run-ins with the law to make a case that the other is not suited for elected office.
And of course, just look in our own backyard, in the ol' 77th Assembly District. Brett Hulsey recently started accusing Dianne Hesselbein of being a flip-flopper on transportation and environmental issues. Specifically, Hulsey and his supporters have accused Hesselbein of "flip-flopping" on the purchase of the Ice Age Trail, a proposal that Hesselbein voted against last year but supports this year, now that the county has negotiated a considerably lower price for the land acquisition.
Negative attacks need to be managed much more carefully in a primary. The voters are better-educated and more passionately political, which means it's harder to pull the wool over their eyes, especially with regards to a candidate from their own party. Moreover, if they feel your ad is misleading, they will likely react angrily and punish you for bringing ugliness into the family.
That being said, in my experience, GOP primaries tend to be uglier than Democratic ones. The best example would be the most recent presidential election. The Republicans wasted no time in savaging each other over abortion, taxes, gun rights...you name it. The Dem primary was a completely different affair. While there was no lack of ill will between supporters of Hillary Clinton and Barack Obama, neither one of them ever ran a full-on attack ad against the other. Sure, there was the "red phone" ad and plenty of criticism on the campaign trail, but the calculation the campaigns made was not to upset the undecideds, who they agreed did not want to see ugliness when choosing between two party sweethearts.