I, personally, believe that quote is fabricated - simply because it is out of context for someone in a bloodthirsty mob to call down curses on their own descendents. I think whoever wrote (or edited) the gospel of Matthew added that at some point, out of hatred for the Jews - and not because they killed Christ, but because at the time the Gospel was written, the friction between Christianity and Judaism over doctrinal matters was very intense. Christianity at that time was not quite its own religion, but still a schismatic sect of Judaism, and the Jews were attacking it as heresy, and the Christians were retaliating in whatever manner available to them (such as adding the preceding to their canonical texts).
Just a theory... but as I said, the quote is completely out of context in the circumsatances in which it is claimed to have been uttered. To reiterate, "His blood be on us", while screaming for Jesus' death, makes sense in that context - but to invoke a curse on one's descendents? I don't think so...
But it's too much to hope for a reflexive scribbler like Safire to even have pondered the Gospels enough to analyze them to that extent... it's easier to just come unglued and shoot the messenger.
Only if you actually believe in the curse, which they did not. If you believe your cause is just, it is a perfectly plausible piece of hyperbole, such as "I swear on my mother's grave." And such hyperbolic moments are ALWAYS reserved for high emotional contexts, which is when something nearly outrageous is required to stand out above the general din and focus the argument. And give assurance to a doubting vote that "if we are SO SURE the blood is innocent, you can trust us on this."
People who are responsible but don't want to be will always borrow extra sincerity from the surrounding zealots, and use it as their own.
Which is precisely what happened in the Gospels, in the most brilliant portrayel of a politician catering to zealots ever painted.
If you follow the dynamic of the argument between the Jews and Pilate, (like the movie did) it fits the context perfectly.