There doesn't seem to be an exact answer, but the closest I could think of I found in Google:
At a Jefferson Republican victory rally in October 1802, Pennsylvania was toasted as "the keystone in the federal union," and in the newspaper Aurora the following year the state was referred to as "the keystone in the democratic arch." The modern persistence of this designation is justified in view of the key position of Pennsylvania in the economic, social, and political development of the United States.