You see, Article I, Section 8 of the Constitution, clearly states that "The Congress shall have power to ... coin money, regulate the value thereof, and of foreign coin." But Congress does not coin money nowadays; the U.S. Mint does! This is obviously an unconstitutional and illegal usurpation of Congressional authority!
And what of regulating the value of foreign coin? The price we pay for euros and British pounds isn't set by acts of Congress, as the Constitution requires. It's time to take back our country from the clutches of the currency traders!
Have I convinced you? Only if you were anti-Mint and anti-currency-trader in the first place, perhaps. Just as you'd have to already be opposed to the war in Iraq to believe the following:
The Iraq war is in direct violation of the United States Constitution.
It's the same ridiculous argument as above; argue that a piece of Article I, Section 8 is being violated, while conveniently ignoring that the last paragraph of that section states that Congress has the power to "make all laws which shall be necessary and proper for carrying into execution the foregoing powers".
It's a nice try, but good luck getting the Supreme Court to agree.