I believe the answers are:
The Bull Moose party was formed by a former Republican
President and Jane Adams.
A third party inevitably split the vote of the leading party.
The majority of votes in united states tend to fall to either one of the two dominant parties (either democrats or Republican).
Since the 1860, there were barely any third parties that could shaken the dominance of these two parties. During the election, the third parties tend to get very little vote, and eventually have to give it all to one of the two dominant parties in exchange of a little influence within the government.
