This study proposed a novel approach to classroom debates, in which chatbots that are able to engage in argumentative dialogues are adopted to facilitate students' debate preparation. The approach comprised three stages: first, students interacted with a chatbot named Argumate to help them generate ideas; second, students discussed the ideas with their group members; third, students participated in debates with other groups. The study investigated students' behavioral, cognitive, and affective engagement in the approach using multiple data sources, including chat logs, audio recordings of group discussions and classroom debates, and students' written reflections, collected from four debate groups involving 24 students. The results showed that the students fruitfully generated ideas through interacting with the chatbot in the first stage, but their uptake of chatbot-suggested ideas in the second and third stages was not sufficient. Cognitively, the students adopted various strategies for sharing, synthesizing, and selecting their self-proposed and chatbot-suggested ideas when preparing for debates. Affectively, the students generally reported positive attitudes toward the integration of the chatbot into debate preparation because it suggested inspiring ideas, provided an interesting approach to debate, and created a stress-free learning environment. These findings provide insights into the use of chatbots to enhance classroom debates.