That is such a ridiculous mantra.
Taken to extremes Republicans can lie, cheat, steal, murder, commit treason, and stab others in the back and we would be admonished not to speak ill of them.
What happened to tell the truth & shame the devil?
There is a difference between speaking inconvenient truths and slinging insults. I think that was Reagan’s point, not covering up behavioral deficiencies of primary opponents.
“I can do better, and here’s why” is a good policy.