Never-Trumpers are playing by Republican rules and being judged according to Republican standards
The Lincoln Group, arguably the most prominent PAC run by Republicans who oppose Trump, is running an ad that would likely spook Democrats.
Calling those who wave the Confederate flag treasonous "losers," and tying them to Donald Trump, is an unapologetic embrace of the culture wars that Republicans have leveraged for electoral advantage for a couple of generations. The folks at The Lincoln Project aren't worried about alienating Southern truckers or the Lynyrd Skynyrd crowd or Dukes of Hazzard fans.
This was the same group that has run ads calling out Trump's tendency to jump between belligerence toward and flattery of the Chinese government, his slurred speech, and directly blame the deaths of 100,000 Americans on Trump's negligence and incompetence.
NeverTrumpers are not at all popular with the left because they were engaged in savage campaigns marked by more subtle dogwhistle versions of Trump's naked bigotry, and used their PR skills to sell America on a disastrous and illegal invasion of Iraq, among other crimes. But that ferociousness is what makes them such a thorn in Trump's side.
They're playing the same game that they've always played when they worked to advance Republican causes--with vicious, no-hold-barred attacks on Trump's character. That's been the norm since Newt Gingrich first started playing to C-SPAN's camera's during the Republican Revolution of the mid-1990s. This is one of the rare occasions when those attacks are accurate.
They're also unconcerned about mainstream pundits taking to the fainting couches over their lack of civility. Just as they're playing the game according to Republican rules, they're being judged by Republican standards. Having jettisoned any last remain norms of factual accuracy. decency or propriety, conservatives are expected to offer no quarter in their campaigns and the media no longer try to enforce them. When Democrats throw a low punch, it raises eyebrows because that's not how they're expected to operate; when Republicans do it, it's a man-bites-dog story.