If VP Becomes Acting President When President Is Temporarily Disabled, What Happens to VP's Tiebreaker Vote?

Looks like it goes away, for however long the VP acts as President.

The Vice President can break a tie in the Senate, which is especially important when the Senate is split 50-50, as it is now. Say a President gets ill enough that he recognizes that he's becoming temporarily incapacitated (or the Vice-President and the majority of the Cabinet so recognizes, for instance if the incapacitation comes on suddenly). The VP would become Acting President; but would she still be able to cast the tie-breaking vote in the Senate?

The Twenty-Fifth Amendment provides, in relevant part,

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 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, transmit 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. [Details for handling disputes about this omitted. -EV]

And the body of the Constitution provides:

The Vice President of the United States shall be President of the Senate, but shall have no Vote, unless they be equally divided.

The Senate shall chuse their other Officers, and also a President pro tempore, in the Absence of the Vice President, or when he shall exercise the Office of President of the United States.

This suggests that, when the VP "exercise[s] the Office of President of the United States," she doesn't exercise the office of "President of the Senate," and therefore lacks the power to cast her "Vote" in the Senate when the Senators are "equally divided." And the 1985 Office of Legal Counsel opinion agrees:

The Twenty-Fifth Amendment does not require a Vice President to relinquish the office of Vice President when he becomes Acting President because of a temporary Presidential disability; in fact, the Amendment and its legislative history clearly contemplate that the Vice President will continue to serve as Vice President during and subsequent to the Presidential disability. See 1965 House Hearings at 87; S. Rep. No. 1382, 88th Cong., 2d Sess. 11-12 (1964). The Vice President would, however, lose his title as President of the Senate. See 111 Cong. Rec. 3270 (remarks of Sen. Saltonstall); J. Ferrick [likely referring to J.D. Feerick -EV], The Twenty-Fifth Amendment 199 (1965).

Here is the Congressional exchange to which the OLC was referring:

Mr. SALTONSTALL… Under the Constitution, the Vice President is President of the Senate, but if he became Acting President under this amendment, he would no longer be President of the Senate, but the President pro tempore would become the President
of the Senate. Is that correct?

Mr. BAYH. That is correct.

Mr. SALTONSTALL. The Vice President would become Acting President and thereby lose his title as President of the Senate. Is that correct?

Mr. BAYH. That is correct.

Sen. Birch Bayh was apparently the principal author of the Twenty-Fifth Amendment, so his position is likely to carry some weight (not binding, but persuasive). And it seems that he and the OLC were implying that the VP would also lose her tie-breaking vote, since that comes only from her position as President of the Senate. I e-mailed Prof. John Feerick to confirm this, and he agreed.

Now if the President dies or resigns, the Vice-President actually becomes President, and a new Vice-President would then presumably be nominated and confirmed by a majority vote of both Houses (though the Senate being 50-50 might require a good deal of compromise on this score). But if the President is merely temporarily incapacitated, there's no provision for choosing a new temporary Vice-President / Senate tiebreaker. That wouldn't be a serious problem for short incapacities (for instance, when a President is in surgery), but might be for longer ones.

Now I suppose one could argue that the Senate President pro tempore would be able to cast two votes, when the VP is Acting President of the U.S.: One in his capacity as Senator, and one in his capacity as Vice-Vice-President. But apparently that has never been the understanding. (Recall that, before the Twenty-Fifth Amendment, when a President died in office, the VP became President and the office of VP remained vacant; there were many years during which this was so, and apparently there was just no tie-breaker then.) [UPDATE: Seth Barrett Tillman reports that such multiple voting in colonial and state offices did happen in 1700s America; but apparently that didn't end up being adopted as the custom in the U.S. Senate.]