George C. Wallace split the anti-Democratic vote in 1968 and almost elected Hubert H. Humphrey. Back in 1935, a poll commissioned by the Democratic National Committee showed that if populist Huey Long ran for President, he could take enough votes to deny Franklin D. Roosevelt re-election. FDR responded by moving left, to co-opt the Long vote. We'll never know what would have happened in the election, because Long was shot later that year. In fact, however, third-party candidates rarely do well enough to determine the outcome.
Voters understand that.
They know that when they vote for a third party, they're helping to elect the candidate that they like least. Conservatives who voted for Bob Smith would be helping to elect Al Gore. None of them alters the outcome of the race. They just take more votes from the front-runner and shrink Bush's lead to between 10 and 14 points. Only one potential third-party candidate has the potential to pull together a coalition of conservatives and populists.