According to Cambridge Dictionary, protest means
to say something forcefully or complain about something
to show that you disagree with something by standing somewhere, shouting, carrying signs, etc.
In British English, the first meaning is generally transitive, and the direct object is a statement of fact, for example:
He has always protested his innocence
He protested that he had been treated unfairly
In British English, the second meaning is generally intransitive and it is normal to use a preposition like about or against to describe in general terms what somebody is complaining about.
He protested that the war in Iraq was a waste of money and manpower. -direct object is a statement of fact
He protested about the war in Iraq - preposition- general description
He protested against the war in Iraq - preposition- general description
Clearly, plenty of people regard Trump as a general description of something to complain about, but Trump could never be regarded as a statement of fact.
So, in British English, you could say
He protested that Trump was a racist
but you can't say
He protested Trump
Usage in the United States is somewhat different. Looking at the entry in Merriam-Webster, you will find this definition for the transitive form:
to make a statement or gesture in objection
He protested the abuses of human rights
Again, we have the idea of making a statement, but if you look at the associated example, you will see that the object is not a statement but a description in general terms of the issue: something that in British English would have to be linked by about.