This section is from the book "The Constitutional Law Of The United States", by Westel Woodbury Willoughby. Also available from Amazon: Constitutional Law.
A criticism that may be made both to the constitutional provision and to the acts of 1792 and 1886 is that the term "inability" to discharge the powers and duties of the presidential office is not defined. In the absence of a definition, who is to determine, and what conditions are to be held to create, an inability on the part of the President to perform his official duties? What is to bedone in case the President is temporarily disabled by sickness or accident, or insanity ? Who is to decide, and by what criteria when this disablement is so serious and so prolonged as to require the appointment of an acting President For the two months preceding the death of Garfield the country had no President able to perform the duties of the Chief Executive.
One further point with reference to the succession to the presidency has been raised. In an interesting article in the American permitting an acting President to decide whether or not there shall be such a special election? If the acting President were to veto such a bill, it is to be feared that the majority in Congress might tie up the whole machinery of the government.
"Let us take another case. Suppose that a Republican President is in office, but that the Republican party is in a minority in one house and has a very slender majority in the other. This condition happened in 1881 under President Garfield. Let us further suppose that the President and Vice-President die; that the Secretary of the State succeeds to the Presidency and that he is bitterly opposed by many members of his party. Is it going too far to predict that the Democratic party might introduce a bill for a special election, knowing its ability to pass it in one house and relying upon assistance from enough members of the Republican party to carry it through the other? Is it not conceivable that the acting President might use all the patronage he controls to prevent the passage of such a bill? Is it not also possible that Congressmen (of course none in the present Congress) might couple requests for appointments of constituents with a gentle intimation that, if made, the acting President need not worry as to the fate of any bill providing for special election.
"It is hardly possible to overestimate the disturbance to the business interests of the country which might arise under such circumstances. The office of President would be held at the will of the legislative body. The power of the executive would be merged in that of Congress. Such a condition would be in hopeless conflict with the principles of the Constitution." Harvard Law Review, XVIII, 191, "The Presidential Succession Act of 1886."
Law Review,6 the author, Mr. Lewis R. Works, points out that the language of the Constitution, strictly followed, would seem to point to, or at least render possible, the construction that upon the death, removal, resignation, or inability of the President, the Vice-President does not become the President, but simply that the powers and duties of the office " devolve " upon him. In Section III of Article I the Senate is authorized to choose a President pro tempore in the absence of the Vice-President " or when he shall exercise the office of the President of the United States," not when he shall become the President. This being so, in cases of disability of the President the Vice-President may by Congress be empowered not to be President, but to act as the President.
The uniform practice has been, however, since the time when Tyler took the oath of office on the death of Harrison, to consider the succeeding Vice-President as becoming the President. Under this practice, however, Mr. Work asks, what, in case of disability, does the late President become, and how, upon removal of his disability, would he again become President? Does the Vice-President cease, for the time being, to be Vice-President, or does he hold both offices ?
 
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