Prominent conspiracy beliefs, such as QAnon or 2020 Presidential Election beliefs, comprise a unique form of conspiracy theories that are often explicitly partisan, "politically instrumental conspiracy theories" (PICTS). These theories can spread rapidly, quickly becoming consensus beliefs within partisan in-groups. But PICTs are not necessarily deeply held, rather serving immediate instrumental partisan needs. We use novel survey list experiments to estimate the prevalence of QAnon and 2020 Presidential Election conspiracy theories in the United States. We find that standard survey techniques likely overestimate the prevalence of PICTs by a factor of ~2. Adherence to PICTs is driven by right-wing media consumption (QAnon), and partisanship (2020 Presidential Election). Over-reporting of PICTs is most common among those with conspiratorial ideation. While the prevalence of PICTs is commonly over-estimated and the beliefs may be ephemeral, they can serve an instrumental role motivating the contemporary American electorate.