From what I remember, Nader's single digit victory took votes away from voters that many have went to Al Gore.
This October 2000 article from the LA Times stays "some political experts say Nader’s single-digit poll numbers--he tends to draw about 4% in nationwide surveys--prove he has no chance of winning, only of detracting from Gore’s voter base." Has that argument been confirmed by historical scholarship? What factors led to Nader gaining such high votes within the context of running a third party candidacy?
Gerald Pomper examined exactly this and and published his results in Political Science Quarterly^(1)
Per the article
...approximately half (47 percent) of the Nader voters said they would choose Gore in a two-man race, a fifth (21 percent) would choose Bush, and a third (32 percent) would not vote. Applying these figures to the actual vote, Gore would have achieved a net gain of 26,000 votes in Florida, far more than needed to carry the state easily.”
George Bush won Florida by only 537 votes (48.847% to 48.838%).
Moreover he states that:
Despite their small numbers, Nader's and Buchanan's supporters provided the margin of victory for Bush. If Nader had not been on the ballot, Gore would have carried Florida and all of the other close states easily, giving him a comfortable electoral total of at least 292. If Buchanan had not been a candidate, the Florida ballot might have been simpler to understand, giving Gore enough votes to win the national election simply by carrying the Sunshine State. Even without Florida, we might speculate--but cannot demonstrate--that an election without Nader would have enabled Gore to campaign in other winnable states (most obviously Tennessee and New Hampshire) and overcome his shortfall of only three electoral votes.”
Now it can be argued that if Nader and Buchanan were not in the race, George Bush would have also changed his strategy to accommodate the altered field. However, given the parameters of your question, I think it is safe to say that third party candidates (particularly Ralph Nader) disproportionately affected Al Gore’s chances of winning the election.