Article IOnly the House may originate tax bills. Hmmm. Since Obamacare was a Senate Bill and it raises taxes, is the bill void or voidable on that basis? (Answer: The Obamacare bill was passed as a House Bill amended by the Senate, despite popular press reports that made is sound as if the Senate initiated its own bill.)
Section 7 - Revenue Bills, Legislative Process, Presidential Veto
All bills for raising Revenue shall originate in the House of Representatives; but the Senate may propose or concur with Amendments as on other Bills.
Every Bill which shall have passed the House of Representatives and the Senate, shall, before it become a Law, be presented to the President of the United States; If he approve he shall sign it, but if not he shall return it, with his Objections to that House in which it shall have originated, who shall enter the Objections at large on their Journal, and proceed to reconsider it. If after such Reconsideration two thirds of that House shall agree to pass the Bill, it shall be sent, together with the Objections, to the other House, by which it shall likewise be reconsidered, and if approved by two thirds of that House, it shall become a Law. But in all such Cases the Votes of both Houses shall be determined by Yeas and Nays, and the Names of the Persons voting for and against the Bill shall be entered on the Journal of each House respectively. If any Bill shall not be returned by the President within ten Days (Sundays excepted) after it shall have been presented to him, the Same shall be a Law, in like Manner as if he had signed it, unless the Congress by their Adjournment prevent its Return, in which Case it shall not be a Law.
Every Order, Resolution, or Vote to which the Concurrence of the Senate and House of Representatives may be necessary (except on a question of Adjournment) shall be presented to the President of the United States; and before the Same shall take Effect, shall be approved by him, or being disapproved by him, shall be repassed by two thirds of the Senate and House of Representatives, according to the Rules and Limitations prescribed in the Case of a Bill.
This section describes an interesting procedure for whether a bill passed by the both houses becomes law. The President may sign the bill or may simply sit on it. If the president sits on it for 10 days and Congress remains in session, the bill becomes law without the president's signature. To prevent the bill from becoming law, the president must veto stating objections, subject to being overridden by 2/3 vote in each house.
BUT, if Congress adjourns within 10 day of sending a bill to the president, and the president sits on the bill without signing or vetoing, the bill does NOT become law. That is not a veto, so it is not subject to being overridden. This is called a "pocket veto."