A recent article features a statement from the Scottish writer George MacDonald to the effect that as no good person would go into politics, anyone who is elected to power will not be a decent human being.
His words may be unwelcome and depressing, but there are many recent examples in both the UK and the US that support them.
Other people mentioned on here have said much the same thing.
This is from Benjamin Disraeli, who saw it all from the inside:
Taylor Caldwell, who had something to say about the causes of major wars, also said this in her historical novel Captains and the Kings:
“...politics and moral ethics never mix. Politics and ethics are a contradiction in terms. An honest politician is either a hypocrite—or he is doomed.”
Although Captains and the Kings is set in the United States and the story starts in the 1850s, much of the material has wider applications.