The fact is, America is not a Democracy. It's a Republic. We have virtual representation by an elected leader, instead of a monarch. That's a Republic. In a true democracy, there would be LOTS of dissenting voices, since every law would come from a mandate from the masses, not from the mind of a single leader or government body like Congress.
However, in a Republic, like the United States (Which is not a democracy) the dissenting voice is always there as a warning for those in power, to remind them that all administrations are limited, and there is someone waiting to kick them out when their term is done, and turn power over to their rival party. The sole party in charge would have no check on their behavior because there would be nothing for them to lose, if there was no one to argue with them. Its only the dread of losing office to the rival party that makes the majority voice do anything good at all. If a dissenting voice can turn enough people away from the lords on top, then the government is replaced by a new administration. That's why our leaders have to be wary of dissent. Caesar used to have a Fool walk behind him to remind of of this. the fool used to whisper in his ear "All power is fleeting."