This week, Republican White House hopeful John McCain denounced George W. Bush's unprecedented use of presidential signing statements. As well he should. After all, it was President Bush's December 30, 2005 signing statement on McCain's amendment to the Detainee Treatment Act that made waterboarding and other acts of torture the continuing policy of the United States.
On Monday, McCain announced that as President, he would reject signing statements altogether:

"I would never issue a signing statement. It is wrong, and it should not be done."

Given his betrayal by Bush over the torture of terrorist detainees, McCain's firm position is unsurprising.
With his signing statement attached to the 2005 Detainee Treatment Act, Bush himself sought to create a legal basis for his administration's past and future criminality. In a nutshell, Bush signed into law a bill he had every intention of continuing to violate.
Bush, of course, had opposed John McCain's torture bill throughout the fall of 2005. But when the House and Senate passed McCain's amendment to the defense authorization bill by veto proof margins, Bush held a press conference on December 15 with McCain, announcing his support for the language explicitly saying that that the cruel, inhuman, and degrading treatment of detainees in US custody is illegal regardless of where they are held.
As the Boston Globe reported, that supposed compromise lasted just as long as it took for President Bush to issue his signing statement two weeks later on December 30. When it comes to what constitutes "cruel, inhuman, and degrading treatment of detainees," the President proclaimed that he indeed would be the decider:

The executive branch shall construe Title X in Division A of the Act, relating to detainees, in a manner consistent with the constitutional authority of the President to supervise the unitary executive branch and as Commander in Chief and consistent with the constitutional limitations on the judicial power, which will assist in achieving the shared objective of the Congress and the President, evidenced in Title X, of protecting the American people from further terrorist attacks.

That shocking presidential power grab, along with Alberto Gonzales' 2005 lies to Congress about the administration's torture policy, served to emasculate John McCain's amendment. It's no wonder he's vowed of future legislation in a McCain presidency that he "would only sign it or veto."