This chapter examines climate policy efforts across 17 US states that joined the Under2 Coalition at different timepoints, analyzing founding members (California, Oregon, Vermont, and Washington), early joiners (Connecticut, Massachusetts, Minnesota, New Hampshire, New York, and Rhode Island), and late joiners (Colorado, Hawaii, Maine, Maryland, Nevada, New Jersey, and Virginia). The analysis systematically compares climate policy adoption, policy instrument diversification, and greenhouse gas emissions reduction targets across these membership categories. California emerges as the dominant climate leader, leveraging its unique Clean Air Act waiver and market power to demonstrate cognitive, structural, entrepreneurial, and exemplary leadership. Other founding members show more variable patterns, with partisan politics creating significant constraints. Early joiners like Massachusetts and New York amplified existing frameworks through the Under2 Coalition membership, while states like New Hampshire represent followership despite early participation. Late joiners generally maintain lower policy effort levels, though some developed ambitious frameworks prior to coalition entry. The findings provide mixed support for the propositions regarding founding member leadership and policy durability.