Why did these groups come together in a single party?

This would be referring to the populist group that was formed out of the interest of farmers both in the west and south whose product had low prices and there was a need to have a strong political presence that can bring reforms that would benefit these people. They went on to form the people's party because they believed that the parties other parties which would be the republicans and the democratic did not have a voice of their own because certain vested interest by the rich and bank owners were being served. So the way forward was to form their own party.

The re-election of the president abraham lincoln.