Ian, yes, whoever gets the most votes wins, even a write-in candidate. But no "third party" candidate has ever won, and I don't think one ever got more than 20% of the vote, in the 20th century anyway.
Actually, it is a more complicated than just the popular vote. The candidate who wins the most "electoral" votes wins the presidency. There are 540 (I think) electoral votes that are parcelled out among the states based on the decennial population census. Whoever wins the popular vote in a state gets ALL of that state's electoral votes. Therefore it is statistically possible for a candidate to win the popular vote but lose the electoral vote count. For example, if California, New York and Texas each had 100 electoral votes, and if Gore won by just one popular vote in each of those three states, he would win the presidency even if he lost every person's vote in all the other 47 states. Therefore, it is crucial for the candidates to win the populous states in order to get the winner-take-all electoral votes.
Reading this board will give a really distorted and one-sided view of the closeness of the race. It is very close. There are groups that, in the aggregate, traditionally have voted for the Democrat candidate for the past 50 years: minorities (especially African-Americans), women, Jews, unionized labor, lower income groups, and artsy and academic types. Unfortunately for Bush, these voting blocs are particulary prevalent in the populous states with large electoral votes--New York, California, Ohio, Michigan, Pennsylvania, Florida. Bush has an advantage in Florida, where his brother is governer, and in Texas, where he himself is of course governor.
Glenn