By Paul Reynolds BBC News Online World Affairs correspondent

George Bush's nightmare is that there will be another attack inside the United States on his watch - and that he will be blamed for not doing enough to prevent it.

After 11 September, nobody can say that a threat does not exist. In fact, Osama Bin Laden's spokesman has in the last few days repeated that further attacks will take place.

The US Government has no more important mission than protecting the homeland

George Bush

But trying to assess where and when they will take place is almost impossible.

Hence the sweeping nature of Mr Bush's Homeland Security plan.

It's the blueprint the President wants followed as he asks Congress to approve a new Homeland Security Department much more powerful than the one he set up after 11 September. Hearings are currently going on in a Congressional Committee.

"The US Government has no more important mission than protecting the homeland from future terrorist attacks", said Mr Bush.

There are three guiding principles:

• Stopping more attacks
• Reducing America's vulnerability
• Minimising any damage

Fantastic scenarios

The plan tackles the problem on several levels - intelligence gathering and warnings; security of borders and transport; domestic counter terrorism; protecting critical infrastructure; and faster response.

Mr Bush has upset some civil liberties groups

One proposal is to set up "red teams" to act as the enemy, seeking weak spots and organising theoretical attacks.

Scenarios once thought too fantastic even for a film will now have to be studied. The skill and imagination displayed by Al-Qaeda will have to be matched.

There is a risk in all this of creating a kind of suppressed panic. Already, there is a certain level of fear.

The US Government believes that it does not have much choice

The language of the document is almost apocalyptic: "A new wave of terrorism, potentially involving the world's most destructive weapons, looms in America's future".

By this, of course, is meant nuclear weapons. It doesn't get much worse.

And even if it did, there is a plan for that, too. Key workers will be vaccinated against smallpox and supplies stockpiled for everyone else. Smallpox is supposed to exist only in laboratories in Russia and the US. There is no confidence any more about its security.

But the US Government believes that it does not have much choice.

A million spies

To defend one place, it has to defend every place. That is difficult. That is expensive. And success is not guaranteed. But the government thinks it is necessary.

More protection against biological attacks is planned

Another danger is to civil liberties. Already the American Civil Liberties Union (ACLU) has complained about a proposal to recruit a million domestic "spies" - people who would act as informants on possible terrorists.

The thinking behind it is that Al-Qaeda cells of the type which hid in American society before 11 September would no longer find it so easy to operate.

But the potential for mistake and mischief is obviously huge.

Overreaching?

Will fear and loathing take the place of the pursuit of happiness in the United States?

Congress, traditional restraint on the executive branch of government, is keeping a close watch on the new plan.

Already some Congressional committees are showing signs of resistance to some of the ideas. They feel that the administration is overreaching. They don't want the Secret Service in the new Homeland Security Department or the Coast Guard, for example.

Mr Bush might not get everything he wants.