The problem with being a known habitual and reflexive liar is that the liar can't effectively counter the lies about themselves with any credibility.
There comes a point after so many lies, both large and small, that the liar's enemies can say anything with just a grain of truth and it's accepted as fact over the objections and denials of the one who can't be counted as a reliable source for honest, truthful information. At that point there's little daylight for reasonable doubt.
Donald Trump is the case for just such a shameless liar. Even if he didn't say disparaging words about service members, it does sound like something he would say, so the benefit of doubt is so lost he may as well have said them.
This is the house that Trump built. These are the lies that make up the house that Trump built. This is the bed that's within the house that's made up of lies that Trump built. May he never find rest or comfort the remainder of his life.