Really, at the end of the day it all comes down to this. Gore lost a close election, but unlike others who had lost close elections in the past, Gore refused to accept the results and exploited known flaws in our election system to try and overturn the result. Nixon could have done what Gore did, but chose not to for the good of the country. Gore simply refused to do so based upon his belief that he was entitled to power. In doing so, Gore announced the new democrat strategy for every election, namely, to never accept any result that didn't elect a democrat.
...and when it became clear that that wouldn't work, he kept going in a clear attempt to undermine the new President.