This Steyn column is kind of all over the place, but he does point out that Thomas Jefferson never said "Dissent is the highest form of patriotism."
The real source? Well, it looks like Howard Zinn said it first, in 2002. Somehow I doubt that our senators, even Democratic ones, will be uttering the phrase, "As Howard Zinn once said..." any time soon. And that might say something about the truth and logic of the quotation.
Personally, I think that being a mime is the highest form of patriotism. Think about that for a while.