What did the populist party hope to accomplish by challenging the republicans and democrats on a national level

National exposure--the Populist Party was centralized to the Midwest and rural Southern areas. 

The Populist Party had won some local and state elections in agricultural areas, however they lacked national exposure to their demands. Being many of their wants would impact workers as well, they wanted to spread their party platform. Eventually, the Populists would join with the Democrats to make a run for the 1896 election.