I know a presidential veto on a law (or bill, since they didn't sign it) can be overruled by a two-third vote in both houses. Is the same, or a similar process, available to overturn a pardon?
No (with one exception, see bottom), there is not currently any limits to the power of a Presidential Pardon. Article 2, Section 2 of the Constitution grants the power:
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 the United States, except in Cases of Impeachment.
Those are the only limits placed on this power:
- For offenses against the United States (Federal crimes, not State crimes)
- Except in cases of impeachment
The judiciary or legislative branches, according to the constitution, do not have any granted power to override or veto a Presidential Pardon. There have been a few controversial pardons taken on by Presidents:
- Pardon of Richard Nixon by Gerald Ford
- Caspar Weinberger, pardoned by George HW Bush against congressional opposition
- Marc Rich by Bill Clinton
- Chelsea Manning by Barack Obama
Just before Clinton left office for Obama, he granted over 100 pardons including one to his half-brother Roger Clinton Jr.. Andrew Johnson pardoned almost all Confederate's for crimes against the United States, which was very controversial at the time.
Additionally this power (granted under State Constitutions) also extends to State-Level crimes for Governors of the respective states where this power is granted. These also do not have limits and have been used quite often.
"A pardon is a deed, to the validity of which delivery is essential, and delivery is not complete without acceptance. It may then be rejected by the person to whom it is tendered; and if it is rejected, we have discovered no power in this court to force it upon him."
This could probably be extended to State Pardons as well.
Legislation is, as the terminology suggests, primarily a function of the legislative branch of government. The veto is a limit on that power. The fact that the veto can be overriden by the legislature is a reflection of the fact that the legislature is the ultimate authority in legislation. The president, on the other had, is the head of executive branch, which is in charge of executing, i.e. carrying out, legislation. The pardon is essentially a refusal to execute legislation in a particular case, and is thus within the purview of the executive branch, and not subject to being overriden by the legislature, nor can subsequent presidents override it.