Democrats are those without power

Most political parties, when they are not in power, criticize the government of being undemocratic, favouring the wealthy and well-connected, ignoring the weak and poor. They demand more tolerance for dissenters, that the government listens to the 'people', distribute the wealth.

When the same people are in power, however, they suddenly (or gradually, for those with a bit more self-awareness) discover a different perspective. Matters have to be considered in the context of the bigger picture and all the information, which only those in power are in possession of. The people out of power have narrow interests and limited information, hence cannot be trusted to make decisions that is best for the community as a whole.

Democrats have a habit of becoming autocrats when they are in power.

The same is true of so many other unequal power relationships: party members vs leaders, employees vs supervisors, small countries vs superpowers, ...

There are exceptions, of course. And I know personally some very notable ones. But it is the rarity of the democrat-in-power that proves the rule.