In order to keep political party bosses from controlling which candidates were chosen to run for office, Progressives pressured state legislatures to call for

Primary Elections

Explanation:

Prior to 1912, party bosses called the shots as to who was fronted for elections in the party. But after the Progressives party was formed in that year, they highlighted the fact that this type of nomination by the bosses was not representative of the people's choice.

So they proposed the start of primary elections which would involve a lot of people from the public in choosing a candidate who would represent the party in the presidential elections.  

i believe it is capitalists.