Dallas was United States attorney for the eastern district of Pennsylvania from 1801 until 1814, a period marked by bitter struggles between the DemocraticRepublican factions in the state, in which he took a leading part in alliance with Governor Thomas M ` Kean and Albert Gallatin, and in opposition to the radical factions led by Michael Leib (1759-1822) and William Duane (1760-1835), of the Aurora.