In a democracy, how can we ensure constructive criticism by the opposition instead of mindless opposing for the sake of it?

By not tolerating mindless opposition from our own side.

If we held our representatives accountable for making arguments supported by facts and logic and based on constitutional principles, that would put a huge damper on much of the foolishness going around today.

Stop unamerican propaganda designed to pit Americans against each other because of a disagreement about anything making them an evil member of the lib tribe.

When the opposition are brain dead racists, we must remove them, as they are the flesh eating virus of democracy.

The first thing you'll have to do is stop thinking that the opposition is doing so mindlessly. That's your arrogance speaking.