Why was John Kerry able to carry New Hampshire?

[Schiff for Senate]

Because he was a New Englander. There was also less focus on economic issues in 2004 than in 2000, which would hurt the GOP in NH, since NH voters are generally socially liberal but economically conservative. In 2000 George W Bush was genuinely seen by some as a 'compassionate conservative,' and won some residual support from moderates (including some Bush 1988/Clinton 1996 voters), but in 2004 the nation was more polarized, and even the remaining few socially liberal voters who had helped put Bush over the top in 2000 reverted to Kerry, since Bush had revealed his true colours as a complete conservative. And also the Nader effect, to a lesser extent - progressives in 2000 were unsatisfied with both Bush (a conservative) and Gore (associated with the nearly centrist Clinton), and had the perfect alternative - a progressive candidate from a new, progressive party. In contrast, in 2004 the Green Party was not as new or glorified, Kerry was more liberal, and he was able to unite peace doves against Bush, including progressive Nader 2000 voters. And NH didn't shift that hard to the left - interestingly, all of NH's neighbours shifted further to the left in 2004  other than MA (Kerry's homestate), where Bush decreased his margin of loss by 2% (and despite this, MA was Kerry's strongest state - the only one where he crossed 60% of the vote).