NJ went from 8% more Democratic than the nation in 2000 (Gore 56%) to 6% more Democratic than the nation in 2004 (Kerry 53%) to 4% more Democratic than the nation (Obama 57%).
Backing up, NJ increased to 8% more Democratic than the nation in 2000 (Gore 56%) from 4.5% more Democratic in 1996 (Clinton 54%), and there from marginally more
Republican than the country as a whole in 1992 (Clinton 42.95%, with Bush well overperforming.)
1992: 0
1996: D+4.5
2000: D+8
2004: D+6
2008: D+4
What I see here is that 2000 is the odd man out here, a high water mark for Dems that is uncharacteristic of NJ.
