A third party would only be successful in securing a Democratic win if the Republican candidate sucks. Why is it always the third party that gets the blame and not the sorry Republican in the race?
Do you actually believe Ronald Reagan would have lost either of his elections if a third party candidate were running?
Abraham Lincoln was somewhat of a third-party candidate in 1860. I seem to recall him winning that year. Former President Teddy Roosevelt came very close, IIRC, in 1912.