How is the growth and institutionalization of political parties shaped by access to public office? Analyzing a natural experiment from Mexico, in which a political party uses lotteries to select candidates for national public office, this paper presents new insights into how access to elected office shapes the institutionalization of new parties. I find that MORENA was able to build local party networks through access to office (around elected legislators) that subsequently mobilized new voters for the party. More specifically, I show that the party was able to organize more successfully and grow its party membership base in localities that at random received access to public office through an elected legislator from the same locality. The findings highlight the importance of access to state resources for the success of new parties and have important implications for debates about democratic representation and accountability.