Who becomes President if Obama dies, resigns, or is removed from office?
"While the Constitution, as amended by the 25th Amendment (230KB), provides that the Vice President will succeed to the nation’s highest office on the death, resignation, or removal from office of the President, it delegates authority for succession beyond the Vice President to Congress. Over the past two centuries, Congress has exercised its authority in three succession acts, in 1792, 1886, and 1947. It has also added to, revised, and clarified the succession process through Sections 3 and 4 of the 20th Amendment, proposed by Congress in 1932 and ratified by the states in 1933, and the 25th Amendment, proposed in 1965 and ratified in 1967. The Succession Act of 1947 (373KB), as amended, and the two constitutional amendments currently govern succession to the presidency."
* "[O]n March 9, 2006, the President [George W. Bush] signed the USA Patriot Improvement and Reauthorization Act of 2005 into law (H.R. 3199, Representative James Sensenbrenner, P.L. 109-177, 120 Stat. 192). Title V, Section 503 of this act revised the order of presidential succession to incorporate the Secretary of Homeland Security as 18th in the line, following the Secretary of Veterans Affairs."