- Changing demographics, as stated above.
- An end to the violent crime epidemic of the 60s-80s. Republicans were able to maintain control of the State Senate into the 21st century by running on an tough-on-crime, anti-tax platform.
- The spending policies of the Whitman-DeFrancesco era also cost the state party a lot of credibility on fiscal issues. Republicans were anti-tax, but supplemented debt for revenue instead of cutting spending.
- The national party emphasizing social issues, though I think this is overstated as Bush II was competitive in 2004 and the state has a huge conservative Catholic population.
With all of this said, be careful pegging NJ as a strongly Democratic state 1992-2012. Clinton and Obama had strong personal appeal here, but Republicans continued to win down-ballot and most suburban districts/towns. Republicans won the popular vote for State Senate in 2013 and very nearly took control of the chamber.
I might argue it's only entering truly Safe D territory post-Christie.
NJ swung to Bush in 2004 because of terrorism and especially considering the Osama tape released before the election. Because of 9/11 there was a substantial swing in NJ-NY suburbs to Bush especially amongst Italian-americans of which many had family members that worked in NYPD or Fire Departments and after 9/11 for a long term their number one focus was terrorism.
