Interestingly, my dictionary gives the following (amongst other definitions of course):

> possessing supreme or ultimate power: in modern democracies the people's will is in theory sovereign.

Your dictionary is wrong.

For example, in the UK Parliament is sovereign, not the people. That's effectively how it is in representative democracies or republics where there is no fast representative recall mechanism.

The U.S. Constitution doesn't say that Congress is sovereign, but the effect is the same. For that matter, the U.S. President has a great deal of power.