We also won Florida by only 537 votes, thanks in part to the infamous butterfly ballot, which resulted in thousands of "Jews for Buchanan".
But mainly I think through fraudulent disfranchisement by a process to identify alleged former felons, ineligible to vote by Florida law, who were in fact not felons in many cases. We know that the private firm Jeb Bush hired to do the list-making objected to orders from the state government to broaden their slanderous accusations because they feared legal liability (and a corporate bad reputation for unreliability) by the criteria Secretary of State Harris retroactively imposed on them, which they knew and explained would create egregiously high numbers of false positives. They demanded, and got, orders in writing to proceed with what they knew were indefensible methods.
This alone more than accounts for the alleged Bush victory margin, by at least an order of magnitude.
Florida was stolen, pure and simple. And without it, Bush could not win.
Mind, I for one had Florida written off before the election showed otherwise as a Bush slam dunk just knowing they'd elected Jeb Bush governor two years before. Florida by that kind of reasoning should have been a solid and indisputable majority for Bush--the fact that it was jittering right on the red line of indeterminacy is a damning enough indictment of the alleged popularity of the Republicans in the period, at least in potentially volatile Florida itself.
The Republican habit of leaning on various layered forms of voter suppression should always be factored in to any claims of legitimacy on their part, and we need not look after 2000 for other examples. It was something they were found guilty of again and again in courts of law.
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None of which invalidates the evident fact of a genuine shift toward supporting Republicans and abandoning Democrats across the nation, only raises some questions as to the magnitude. Indeed the Republicans had momentum and as noted above a big part of it was that it was "their turn." Fatigue in supporting the party that has had control of the White House for some considerable time is a major fact of American politics; no party should ever count on winning three terms in a row, and definitely not four. I'd explain that as a mix of several factors--one being that there are political problems that can be solved fairly easily without upsetting powerful opposition too much but that a faction that came in on certain premises has a hard time addressing, while others are harder to solve and require a fresh political mandate to overcome deeper entrenched opposition. A new administration of one party can tackle some of the latter and have easy wins tackling some of the former too, then they have exhausted the easy ones and accumulate problems the opposition has more credibility in addressing.
But I do think a certain old boy (nowadays more and more also old girl) network mentality about the nature of the American system held by ruling elites is that the system works by a revolving door in the Presidency, and thus opposition to a rival party winning tends to weaken after one has held power for two terms or more, while various more or less cynical actors such as corporations tend to suppose it is the other party's turn now. Bush Jr had an inherent advantage, one his father suffered from being on the opposite side of, and 2000 was his to lose. The fact he squeaked by with barely enough EV and a crucial part of them egregiously stolen with a whole series of acts of criminal mendacity was pretty telling in that context.
In retrospect we can see that Bush Jr needed everything that did favor him and it was not sufficient to win honestly even so. All factors mentioned--Gore's poorer charisma than Clinton's, his more elitist self-presentation (perhaps these are one and the same thing? but one can be a charismatic elitist or an uncharismatic populist, so no not really I think), clearly a factor. General shift of the public rightward overall, and especially so in the South, another. Partisan fatigue based on the Clinton administration playing out its hand of easy wins and accumulating baggage that looked amenable to a Republican solution, combined with it simply being the Republicans' turn in elite perceptions--all of these aided Bush, and yet he could not win either a popular vote plurality nationwide nor Florida's crucial EV without a criminal thumb on the scales there.
I do think the Gorebot should have taken a tack in the Florida dispute publicizing the degree to which Jeb Bush's voter purge blocked honest citizens from voting and the Democrats should have backed a class action suit by purged non-felons for slander (or libel, whichever legal category applies better) in conjunction with deprivation of civil rights, and that a case of criminal conspiracy to disfranchise based on racial bias alone (the courts don't much care about partisan bias, but racial discrimination is more legally and politically radioactive) and used plain legal grounds to reject Florida's allegedly legitimate EV.
Of course had SCOTUS not given Jeb legal cover to assert the official claim of a narrow victory, the state legislature, Republican dominated, could claim that the legal method of choosing electors having failed due to an indeterminate PV outcome in Florida, it fell to them to appoint electors, which would push Bush over the top--but highlighting the criminal shenanigans that
manufactured an indeterminacy where none would exist with an honest process.
Instead he took the position that legal processes had been followed, rejected an appeal from members of House and Senate to dispute the Florida claim, and thus obscured the general issue of dubious practices in American elections that haunt us to this day--properly so, as abuses continue. Unresolved.
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It remains a fact Gore performed less well than Clinton, and if we say "but his popular vote percentage was higher!" surely that has a lot to do with Ross Perot challenging strongly in both '92 and '96 but giving up by 2000. Then Nader took it up instead but never came close to Perot's numbers even in '96. We've covered a constellation of different reasons, and on one hand, had electoral propriety held in all states, we can be confident Gore would win and by a less narrow margin of EV than Bush claimed as the outcome of crime. So just as Bush Sr rode on Reagan's coattails to a third Republican term, Gore would win in an honest process--but of course more marginally than Bush Sr did in '88, and very likely have failed of reelection in 2004. On the other, criminality limited to a marginal degree that had plausible deniability in our mainstream media culture was sufficient for Bush to win, because Gore was indeed weaker than Clinton.