According to these seasoned campaigners, the answer is: yes, stupid.
In short, attack ads work better than positive messages because:
1. People remember them better.
2. People believe them more.
3. As a result of (1) and (2) their impact is more immediate.
I found these three great quotations about negative political advertising in a book called Crowded Airwaves: Campaign Advertising in Elections, that whilst a little old still provides some really interesting insights into the subject.