Negativity

PAC spending in the last week:

Committee	Entire Cycle Total	Last Week Total	Last 24 Hours Total	Supported	Opposed	Super PAC
Republican National Cmte (R)	$9,485,456	$9,485,456	$0	$0	$9,485,456
Priorities USA Action (D)	$14,900,294	$1,244,192	$0	$0	$14,900,294	x
Restore America's Voice PAC (R)	$582,952	$111,900	$0	$0	$582,952	x
America's Next Generation(R)	$106,500	$42,500	$0	$0	$106,500	x
Moveon.org (D)	$367,171	$33,552	$0	$5,400	$361,771
National Rifle Assn (R)	$110,952	$31,459	$0	$0	$110,952
National Right to Life Victory Fund (R)	$163,260	$6,786	$0	$0	$163,260	x

Candidate money spent last week:

Candidate	Entire Cycle Total	Last Week Total	Last 24 Hours Total	Supported	Opposed
Obama, Barack (D)	$27,761,536	$9,678,101	$0	$2,852,327	$21,064,674
Romney, Mitt (R)	$32,603,333	$1,277,744	$0	$7,285,760	$25,161,173

I added rough party affiliations for the PACs, but it's worth noting that Moveon.org spent some money in opposition to a democratic candidate for the House of Representatives.
Notice more generally how the money is all piling into the "opposed" column. Source: http://www.opensecrets.org.