Liking what they saw

I was looking for polls on Trump's popularity early in his campaign, and found several by the Monmouth Poll that reported the figures for Republicans separately (including leaners).  The figure shows the percent favorable minus the percent unfavorable:

The first two (blue dots) were before his official announcement; the next two (red) after his announcement but before the debates; the last three (green) after the first debate.  I didn't expect to see much change:  I thought he had come into the race with a pretty strong base of support and maintained it.  But in fact, opinions were mostly negative before he announced (20 percent favorable and 55 percent unfavorable on June 14, 2015), and then became favorable (59 favorable and 29 unfavorable by September 2). 

What led to the change?  The timing suggests that the very early days of his campaign were the decisive period.  I don't have time for a detailed examination, but I looked at the text of his announcement speech.  Some points that struck me:
1.  There was very little discussion of "culture war" issues:  nothing about abortion, same-sex marriage, political correctness, the mistreatment of Christians, the condescension of cultural elites......  The only thing I noticed was a call to "Protect the Second Amendment."
2.  There was nothing that seemed to be an appeal to "white nationalism," even in the broad sense of declining relative numbers and power.  Of course, there was the passage that is most remembered today, about how immigrants from Mexico are "bringing drugs.  They're bringing crime.  They're rapists."  But this was framed as a criticism of foreign governments--"when Mexico sends its people, they're not sending their best." 
3.  There was a lot about economics, mostly standard stuff about the dangers of deficits and the evils of Obamacare.  There was a promise of rebuilding infrastructure, but the implication was that it could be done without more taxes or spending, because he was a businessman who could do things efficiently. 
4.  The idea that America was being "beaten" in competition with other nations was a big theme.  "When was the last time anybody saw us beating, let's say, China in a trade deal? They kill us. I beat China all the time. All the time.
When did we beat Japan at anything? ...  They beat us all the time. When do we beat Mexico at the border?  They laugh at us all the time, at our stupidity.  And now they are beating us economically. The U. S. has become a dumping ground for everyone else's problems. [This was followed by the passage about the people Mexico was "sending"]." As this passage suggests, his major explanation for this state of affairs was the "stupidity" of American leaders.  A secondary one was that the leaders were beholden to donors, and he wouldn't be, because he was rich and didn't need their money.  

So I would say #3 reassured Republicans, and  #4 added a new thing that other candidates weren't emphasizing, but fit with a view that is popular among the public, especially Republicans. 

[Data from the Roper Center for Public Opinion Research]