New online platforms have played a central role in political campaigns over the last decades by allowing small contributors to intervene in local and national electoral politics, and potentially mitigate the capture of the political process by large donors. Their impact has not yet been evaluated. While the focus of the literature has mainly been on large donors or aggregate campaign spending, this paper examines a new form of political participation: small campaign contributions. We build a novel dataset including all the 340 million contributions reported to the US Federal Election Commission between 2004 and 2020. We identify a total of 30 million unique contributors. Due to the particular legal structure of the new online platforms first used by Democrats (ActBlue) and now Republicans (RedWin), the reported contributions include precise information about a majority of the small contributions (contributions under $200). Indeed, while these contributions are hidden when they are made directly to candidates, they are publicly available when made through an online platform. This allows us to differentiate the "large" from the "small" donors, defined as donors who contribute less than $200 during a two-year electoral cycle to at least one committee. We collect unique information on these donors, including their exact address, occupation, gender, and race. Using these data, we first investigate the characteristics of small donors and show the extent to which their behavior differs from large donors. Second, we study the determinants of small campaign contributions and assess the compatibility of observed empirical patterns with strategic vs. non-strategic motives. Our most saturated model, including seat-year and contributor fixed effects, reveals that small donors contribute more often to campaigns out of their district and out of their state than large donors, which participates in the nationalization of local U.S. politics. Third, using Spenkuch and Toniatti (2018)'s identification strategy, we measure the responsiveness of small contributions to TV ads to disentangle the role of demand and supply factors.