Why does he bring it up now? He brings it up because he wants the Patriot Act reauthorized, and because he wants to not be questioned on his authorization of domestic spying on US citizens without a warrant. And he intends to do that by scaring the American public into agreeing. And as Glenn Greenwald notes, "the White House doesn't care how transparent their manipulation of terrorist threats is, because this manipulation is not aimed at our rational faculties."
The Founding Fathers, having won a war to cast off the yoke of monarchy, thus freeing themselves from the tyranny of despotism, were acutely aware of the ways in which their newly won liberty might be lost.
In the Federalist #8 Alexander Hamilton wrote
Safety from external danger is the most powerful director of national conduct. Even the ardent love of liberty will, after a time, give way to its dictates. The violent destruction of life and property incident to war, the continual effort and alarm attendant on a state of continual danger, will compel nations the most attached to liberty to resort for repose and security to institutions which have a tendency to destroy their civil and political rights. To be more safe, they at length become willing to run the risk of being less free.He knew that the fear of external threat erodes liberty, and that in a state perpetually threatened
The perpetual menacings of danger oblige the government to be always prepared to repel it; its armies must be numerous enough for instant defense. The continual necessity for their services enhances the importance of the soldier, and proportionably degrades the condition of the citizen. The military state becomes elevated above the civil. The inhabitants of territories, often the theatre of war, are unavoidably subjected to frequent infringements on their rights, which serve to weaken their sense of those rights; and by degrees the people are brought to consider the soldiery not only as their protectors, but as their superiors. The transition from this disposition to that of considering them masters, is neither remote nor difficult; but it is very difficult to prevail upon a people under such impressions, to make a bold or effectual resistance to usurpations supported by the military power.The Founders foreseeing this, sought to create institutions that would prevent such from happening. The Constitution and the Bill of Rights was the fruit of their labor. It was with an eye to the future and a memory of past injustice that they wrote these documents.
President Bush, instead of upholding and defending these institutions, seeks to undermine them by his use of fear as a political device. This administration has done so by creating a war frame, despite the fact that we are not at war. No matter what anyone might say, 'Terror' is not a state, it has no armies, and it will not sign a ceasefire. It is an abstraction, and declaring war on it puts us perpetually at war with an intangible enemy. Orwell recognized in both Animal Farm and 1984 that such an enemy coupled with fear can be used to justify totalitarianism.
This is not to say that terrorists are not a threat that must be taken seriously. The potential devastation that could be wrought by a fanatic in possesion of a nuclear device is indeed frightening. Which is another reason why it should be unacceptable for an administration to 'cry wolf' in order to gain political capitol.