Is that one, they can become quite popular

And Two, the establishment attacking them often won't damage their support.

Just thinking on this as I see Romney launching a big attack on Trump. It seems to help frame it as the Republican establishment vs Trump, and given that most of the people voting for Trump look to be people unhappy with the establishment, I can't help but feel it is likely to strengthen Trump's support.

There is a lot of stuff coming out lately about Trump's past company bankruptcies, and his inconsistent statements, potential racism etc., but hasn't all this been pretty clear all along? It seems to me that his voters are aware of these, and so pointing it out time and again isn't going to achieve much.

I could be wrong, and maybe consistent attacks like this will eventually make a difference, but it just seems the wrong sort of tactic to me to try and stop such a candidate, but not being from / in the USA, I'm not that strong on US political history so perhaps this sort of thing has worked in the past.