ARTICLE II
Section 1. 1. The Executive power shall be vested in a President of the United States of America. ...
Compare with: ARTICLE I Section 1. All legislative powers herein granted shall be vested in a Congress of the United States, which shall consist of a Senate and House of Representatives. ...
4. No person except a natural born citizen, or a citizen of the United States, at the time of the adoption of this Constitution, shall be eligible to the office of President; neither shall any person be eligible to that office who shall not have attained to the age of thirty-five years, and been fourteen years a resident within the United States.
5. [In case of the removal of the President from office, or of his death, resignation, or inability to discharge the powers and duties of the said office, the same shall devolve on the Vice President. . .]
-- Altered by 25th Amendment:
Section 1. In case of the removal of the President from office or of his death or resignation, the Vice President shall become President.
Section 2. Whenever there is a vacancy in the office of the Vice President, the President shall nominate a Vice President who shall take the office upon confirmation by a majority vote of both houses of Congress.
Section 3. Whenever the President transmits to the President Pro tempore of the Senate and the Speaker of the House of Representatives his written declaration that he is unable to discharge the powers and duties of his office, and until he transmits to them a written declaration to the contrary, such powers and duties shall be discharged by the Vice President as Acting President.
Section 4. Whenever the Vice President and a majority of either the principal officers of the executive departments or of such other body as Congress may by law provide, transmits to the President Pro tempore of the Senate and the Speaker of the House of Representatives their written declaration that the President is unable to discharge the powers and duties of his office, the Vice President shall immediately assume the powers and duties of the office as Acting President ...
Also see the statute on presidential succession:
3 USC Sec. 19 (a)(1) If, by reason of death, resignation, removal from office, inability, or failure to qualify, there is neither a President nor Vice President to discharge the powers and duties of the office of President, then the Speaker of the House of Representatives shall, upon his resignation as Speaker and as Representative in Congress, act as President... (b) If, at the time when under subsection (a) of this section a Speaker is to begin the discharge of the powers and duties of the office of President, there is no Speaker, or the Speaker fails to qualify as Acting President, then the President pro tempore of the Senate shall, upon his resignation as President pro tempore and as Senator, act as President... (d)(1) If, by reason of death, resignation, removal from office, inability, or failure to qualify, there is no President pro tempore to act as President under subsection (b) of this section, then the officer of the United States who is highest on the following list, and who is not under disability to discharge the powers and duties of the office of President shall act as President:
(2) An individual acting as President under this subsection shall continue so to do until the expiration of the then current Presidential term, but not after a qualified and prior-entitled individual is able to act...
... 7. Before he enter on the execution of his office, he shall take the following oath or affirmation:
"I do solemnly swear (or affirm) that I will faithfully execute the office of the President of the United States, and will to the best of my ability, preserve, protect and defend the Constitution of the United States."
Compare with 5 USC Sec. 3331: An individual, except the President, elected or appointed to an office of honor or profit in the civil service or uniformed services, shall take the following oath: ''I, AB, do solemnly swear (or affirm) that I will support and defend the Constitution of the United States against all enemies, foreign and domestic; that I will bear true faith and allegiance to the same; that I take this obligation freely, without any mental reservation or purpose of evasion; and that I will well and faithfully discharge the duties of the office on which I am about to enter."
Abraham Lincoln, 1864: "I did understand, however, that my oath to preserve the Constitution to the best of my ability imposed upon me the duty of preserving, by every indispensable means, that government -- that nation, of which that Constitution was the organic law ... I felt tha measures otherwise unconstitutional might become lawful by becoming indispensable to the preservation of the Constitution through the preservation of the Nation."
Section 2. ...
1. The President shall be Commander-in-Chief of the Army and Navy of the United States, and of the militia of the several States, when called into the actual service of the United States; he may require the opinion, in writing, of the principal officer in each of the executive departments, upon any subject relating to the duties of their respective offices, and he shall have power to grant reprieves and pardons for offenses against against the United States, except in cases of impeachment.
Compare with Article I, Section. 8. The Congress shall have Power To declare War, grant Letters of Marque and Reprisal, and make Rules concerning Captures on Land and Water [Clause 11]; To raise and support Armies, but no Appropriation of Money to that Use shall be for a longer Term than two Years [Clause 12; To provide and maintain a Navy [Clause 13]; To make Rules for the Government and Regulation of the land and naval Forces [Clause 14];
2. He shall have power, by and with the advice and consent of the Senate, to make treaties, provided two-thirds of the Senators present concur; and he shall nominate, and by and with the advice and consent of the Senate, shall appoint ambassadors, other public ministers and consuls, judges of the Supreme Court, and all other officers of the United States. . .
3. The President shall have the power to fill up all vacancies that may may happen during the recess of the Senate, by granting commissions, which shall expire at the end of their next session.
Section 3.
He shall from time to time give to the Congress information of the state of the Union, and recommend to their consideration such measures as he shall judge necessary and expedient; he may, on extraordinary occasions, convene both Houses, or either of them, and in case of disagreement between them, with respect to the time of adjournment, he may adjourn them to such time as he shall think proper; he may receive ambassadors, and other public ministers; he shall take care that the laws be faithfully executed, and shall commission all the officers of the United States.
Section 4.
The President, Vice President, and all civil officers of the United States, shall be removed from office on impeachment for, and conviction of, treason, bribery, or other high crimes and misdemeanors.