The National Rifle Association, for sure.
This information was assembled by the invaluable Institute for Southern Studies. Click the links for specific citations.
The interesting news is that President Obama re-nominated an ATF director the NRA found unacceptable. Might he perhaps be determined to cross the NRA? The proof would be if he made a recess appointment, filling the job when Congress is not in session.
Perhaps also, more vigorous firearms controls may be a pre-requisite for better cooperation with Mexico, where guns acquired in the U.S. are arming murderous drug dealers.