How is the growth and institutionalization of political parties shaped by access to public office? In this paper, I analyze a natural experiment from Mexico, in which a political party used a lottery system to select candidates for national public office, and present new insights into how access to elected office shapes the development of new parties. I find that the party was able to build local party networks by gaining access to office (through elected legislators) that subsequently mobilized new voters for the party. I show that the party was able to institutionalize more successfully and increase its membership base in localities that randomly 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.