It’s actually quite simple, and it has nothing to do with Iraq, or the War on Terror, or the way they acted during the Vietnam war, or any of that.
When Democrats talk about America, they talk about what’s wrong with America, how bad it is, and what they want to do to make it better.
When Republicans talk about America, they talk about how great America is, and what they want to do to make it greater.
Democrats talk about America’s failures (and there are some to talk about, nobody’s perfect) as if they are what defines America.
Republicans may talk of America’s failures, but they treat them as mistakes to be learned from rather than defining characteristics.
Americans, as a general rule, don’t like to listen to their country being run down, especially by its own leaders. As long as Democrats continue to talk America down, they will continue to be seen as unpatriotic, or at least less patriotic then Republicans.