The Origination Clause.
Obamacare is a tax. Tax bills originate constitutionally in the House. Obamacare was written Harry Reid's 'Rat Senate.
In most cases, laws originating in the House virtually always go through a conference process involving a joint House/Senate committee - where a House passed bill gets touched up and is then returned to the House; and etc.
Let me know if I"m wrong.