Real-world tests of the impact of partisan cues on voters are scarce because they require assessing how citizens’ attitudes changed toward an issue from before to after it became politically divisive. During the 2015–2016 New Zealand flag referendums, the leader of the (center-right) National Party and then–Prime Minister, John Key, championed changing the flag—a move strongly contested by the (center-left) Labour Party. Accordingly, we measured New Zealanders’ attitudes toward changing the flag using national longitudinal panel data collected in 2013, before the change was proposed, and again in 2016 at the height of the debate ( Ns = 6,793–6,806). Registered voters who supported the National Party were more likely to shift from opposing to supporting the flag change, whereas those who supported the Labour Party were more likely to shift from supporting to opposing the change. These data demonstrate the powerful impact of partisan cues on political attitudes in a real-world setting.