And so what if it is 'declared' illegal? Who is going to declare this? By doing so, they are admitting their fear of bitcoin and it will only strengthen the cause. By making it illegal, they are reinforcing the ideals that drove the creation of the technology in the first place.
They tried to 'declare' alcohol illegal in the US at the start of the 20th century and look how that turned out.
Comparing the outlawing of Bitcoin to the ratification of the 18 Amendment to the Constitution is a bit of a stretch.
A more accurate historical comparison would be the Gold Reserve Act of 1934 which outlawed the personal possession of gold. In fact, it's still illegal in some states to perform financial transactions with gold.
So, historically speaking, Bitcoin is small potatoes compared to what the US government has regulated/outlawed in the past.