We all deplore it - but, so it seems, it can be extremely effective. That's the reason why so many candidates (and especially their "independent-expenditure only" followers ['SuperPacs']. The most memorable political ads - and probably the most effective - in the last half century have been negative (from LBJ's "Daisy Ad" invoking fear of the possible consequences of Barry Goldwater's 'extremism'...
through the Willy Horton "revolving doors" ad by Bush to undermine the Dukakis campaign in 1988.
C-SPAN recently broadcast a programme on the history and current use of negative political advertising. It is available to view here.