Even if it were only a nannygate scenario, which it isn't, that should be sufficient.
The whole untaxed nanny thing was a tactic started by Republicans, even though many of them were guilty of the same. If a tactic is fair game for you to use, it is fair for others to use against you.
Also, Kerik had politica ambitions before this year. He had the time to clean up any tax problems and do a strenuous background investigation on himself to see what would shake loose. The fact that he didn't prepare himself adequately for this says a lot.
The fact that the White House did not do sufficient research before announcing a candidate says much about them too. The information about the warrant and conflicts of interest should have been easy to find. It certainly seems to have been for reporters.
I'm curious what kind of dirt is found.