the original issue about the nanny, was "outed" by Kerik himself. the subsequent smears of his affairs, his "mob" connections, his stock options, alleged gifts, etc - that's the smear job.
Honestly, I didn't hear about any of this, until after he withdrew his nomination. Not saying any of this is true, there were probably issues either Kerik didn't want brought out, or the administration felt they could not defend. Maybe they were just uncomfortable with the matter.
In any case, he is not up for it anymore. Oliver North would be great. He already had his laundry aired out a number of years ago. That laundry could actually work in his favor--there are certain elements you don't fight on traditional grounds. We'll all just have to wait and see what happens.