Elected incumbents with support from legislative supermajorities have recently been primary drivers of democratic backsliding and subversion in some established democracies. Why and when do established democracies fall prey to such threats? I argue that extraordinary events can give leaders of policy-seeking political parties in polarized societies an opportunity to select populist candidates who help them gain a qualified legislative majority required to dismantle horizontal checks and balances. Such democratic backsliding enables party leaders to lock in their ideal policies. Yet, reduced checks and balances may also enable populist incumbents to subvert democracy completely, thereby sidelining party leaderships in policy making. Impeachment through supermajorities in presidential democracies and votes of no confidence through absolute majorities in parliamentary ones can constitute a credible threat against such populist opportunism. However, uncertainty about the fraction of a populist's loyal adherents among legislators can undermine it. A Markov game shows that party leaders may nevertheless have incentives to gamble on democratic subversion. Polarization between political parties reinforces their incentives to dismantle checks and balances. Heterogeneity of policy preferences within political parties reduces them. It predicts that parliamentary democracies are at higher risk to get dismantled than their presidential counterparts. Strikingly, pure forms of parliamentary sovereignty will survive extraordinary times and remain liberal democracies only if party leaders feature sufficient democratic values. Once presidential democracies have backslidden, however, they are at higher risk of full subversion than backslidden parliamentary democracies.
