Interest groups regularly face decisions about whether to enter new
political venues. We study one such decision: whether groups that lobby the
Supreme Court by filing amicus briefs go on to mobilize for or against Supreme
Court nominees. Drawing on a theory in which groups act where citizen support
covers the cost of acting, we predict that entry depends on the demand for
activism, the cost of entry, and competition from groups that have already
entered. We test these predictions on a population of \nRiskSetGroups\ frequent
amicus filers, following each across every nomination from 1930 through
\nWindowYear\ and estimating discrete-time hazard models of first entry. We
find that groups affiliated with a social movement, groups with recent amicus
activity, and ideologically extreme groups are all more likely to enter
nomination politics. We find no evidence that surges in movement salience or
ideological distance from a nominee increase entry. Our results suggest that
the polarization of interest group activity on Supreme Court nominations is
partly a product of which groups choose to enter.