A favorite grudge against Sen. John McCain among conservative leaders and lawyers is that he sponsored campaign finance reform legislation that they deem to be a violation of the core First Amendment right to political speech. Particularly troubling for them was the non-lawyer McCain's supposedly urging his Congressional colleagues who had reservations about the law's constitutionality to vote for it and let the Supreme Court decide which provisions passed constitutional muster.
Due to the unpopularity of judicial power on the right, this is viewed as not only a bad thing to have said about the particular law, but an inherently dangerous view for any member of the other two branches of government to hold. While the courts have judicial review over the constitutionality of legislation and the executive's interpretation of its statutory and constitutional powers, Congress and the President nonetheless swear to uphold the Constitution* and are obliged to act within the bounds of what they understand the Constitution to mean. Simply abdicating all constitutional understanding to the courts is a failure of duty and is particularly alarming to conservatives who fear kritarchy.
McCain's attitude of legislate it all and let SCOTUS sort 'em out stands in particular contrast to the current Republican president. Although Bush signed McCain-Feingold stating his own doubts of its constitutionality, he did not issue a signing statement of the sort he has with legislation that he deems to encroach upon his unitary executive powers. Evidently Congress's abrogating the freedom of speech is something that the courts can be left to figure out, but Congress's mandating that no funds be spent for a "permanent" U.S. military presence in Iraq is something that Bush must declare unconstitutional immediately. And no expecting "that the courts will resolve these legitimate legal questions as appropriate under the law" when it comes to Congress's prohibiting the administration from establishing permanent bases in Iraq or controlling Iraqi oil resources; establishing a congressional commission to review military contracts in Iraq; protecting contractor whistle-blowers; and putting a 45-day deadline on U.S. intelligence agencies to respond to information requests from Congress' committees on intelligence and armed services. Nope, then it's just "The executive branch shall construe such provisions in a manner consistent with the constitutional authority of the President."
* The Constitution prescribes a specific oath for the president, but not for anyone else. Art. II Sec. 1 states, "Before he enter on the Execution of his Office, he shall take the following Oath or Affirmation: 'I do solemnly swear (or affirm) that I will faithfully execute the Office of the President of the United States, and will to the best of my Ability, preserve, protect and defend the Constitution of the United States.'"
In contrast, Art. I doesn't mention any oath. Art. VI says, "The Senators and Representatives before mentioned, and the Members of the several State Legislatures, and all executive and judicial Officers, both of the United States and of the several States, shall be bound by Oath or Affirmation to support this Constitution; but no religious Test shall ever be required as a Qualification to any Office or public Trust under the United States."
The First Congress's version of this requirement was "I do solemnly swear (or affirm) that I will support the Constitution of the United States." I greatly prefer it to the wordy remnant of Civil War suspicions that is used today: "I do solemnly swear (or affirm) that I will support and defend the Constitution of the United States against all enemies, foreign and domestic; that I will bear true faith and allegiance to the same; that I take this obligation freely, without any mental reservation or purpose of evasion; and that I will well and faithfully discharge the duties of the office on which I am about to enter: So help me God."