Final answer:
The second most powerful official in Washington DC after the President is the Speaker of the House of Representatives, who is second in the presidential line of succession and holds significant legislative and party leadership roles.
Step-by-step explanation:
The second most powerful official in Washington DC after the President, according to the United States presidential line of succession, is the Speaker of the House of Representatives. This position is elected by the entire body of representatives and holds numerous roles: the presiding officer, the administrative head of the House, the leader of the majority party, and an elected representative of a congressional district. The Speaker is second in line to succeed the President in an emergency, following the Vice President, but their power also extends to legislative authority and influence over the House.