1. Most doctors do not belong to the AMA. (About a third are members; many for reasons like making it easier to get a medical license–the AMA provides a letter of “good standing”)–http://www.npr.org/templates/story/story.php?storyId=105393162
2. A large portion of the AMA’s membership are either medical students (who join for the low-cost JAMA subscription, commuter mugs, and other benefits) or retirees.
3. Most of the AMA’s revenues are not from membership. What makes the AMA such an outsized political contributor and lobbying force are things like their ownership of the proprietary CPT coding system (the codes that link diagnoses from the ICD-9 system to billable interventions, e.g., ICD-9 for “high risk sexual behavior” is V69.2, then there are CPT codes for counseling, HIV test, syphilis test, digital rectal exam, etc, etc, so the doctor can bill for those things.) The AMA sells the CPT codes and their associated books, databases, etc; and also incidentally uses these CPT codes as a way of distorting the reimbursement system to favor specialists and sabotage primary care. They also sell mailing lists of doctors for marketers; and so on. Their income far outstrips their actual constituency.
4. To the extent that doctors belong to organizations, they almost all put more energy into their specialty organizations; e.g, I need to renew my dues to the American College of Physicians (internal medicine) and the Society for General Internal Medicine (academic primary care organization for internists), but there is no reason for me to join the AMA.

BTW I saw Obama’s speech while running at the gym at lunch. It is nice to see he is trying to look at all options and just not feed the big insurance companies.