The fact a majority were still good re-electing him doesn't change the fact that a lot of people were furiously opposed to Nixon, and there was much public criticism of his actions in the war, which tainted his legacy. In ancient Rome it was totally different because there was 0% criticism of atrocities by Rome, zero protests, zero public debate about being too brutal, and zero legacy tainting from brutality to Rome's enemies (quite the reverse usually). Nixon won 60% of the voters in 1972, but that means 40% were still against him; and clearly from the protests and mass movements against him (and Vietnam more specifically) there was widespread opposition by many people. In ancient times that number was 0%. ZERO. There were no protests or opposition movements against brutality inflicted on non-Romans. There were also consequences to stuff that happened in Vietnam. Not as many as we could hope for, but numerous court martials and the like happened. Again; there is no evidence of any such thing happening in Ancient Rome.Nixon was overwhelmingly reelected in 1972 in one of the most one sided elections in recent memory. This was long after the public learned about these numerous Vietnam atrocities. If this stuff was really so unappealing to the American public they could have shown him the door. That they didn't tells me a lot about what the public at large thought.
EDIT; and yes, protests are important, they show us what the feelings of society were towards an issue. The Vietnam protests show us a lot of people were against the war. The anti-war movement shows us that. The fact many of the atrocities were hidden by the people committing them shows us the moral climate of the times. The fact that, when some of these atrocities were uncovered, there were numerous court martials etc, shows us the moral expectations of the times. The fact NONE of this happened in ancient times also tells us a lot too. It's pretty basic stuff. Ancients had slave markets because it was deemed moral to do so. We don't because it's not. Keep ignoring reality though.