The Pres shouldn't need to defend himself while he is in office. He has other, more urgent matters for his attention. If the Pres needs defending, it should be exactly his supporters who do it. Before someone abandons such support, consider what the Pres is trying to accomplish and whether there is anyone who better deserves support. Basing it all on one issue is probably not going to be productive; the job is very complicated.
But he's not attending to them either. The President has the biggest bully pulpit in the world. He doesnt have to do the negative work of defending himeself but he should be out there daily in a strong positive way pushing his agenda. Where has he spent his time recently? Touring New Orleans four or five times to make up for not watching TV, telling us that Ms Miers was the best qualified person in the US that he could find to nominate to the Supreme COurt, to say he was sorry to receive Scotter Libbys resignation, to say he cant comment on Rove because there is an ongoing investigation, talking about taking people's tax deductions away from them to come up with a simpler tax, and talking about Bird Flu (half dont believe its an issue and will blame Bush regardless of what he does, other half are scared shitless about it and immediately tune him out). DOesnt sound like a positive program to me.