But here's a more specific point.
Lots of pundits will now say that Trump has upended the playing board. They will say this changes everything. That's not smart. It doesn't change anything. It just makes it all more stupid.
Are non-Trump-supporters now more likely to embrace his candidacy? Not likely. This doesn't make him seem more moderate. It makes him seem more erratic and now desperate.
We are less than three months before the election. And it's not even clear what platform Trump is running on. Is he a white nationalist or a compassionate conservative? Will he keep ranting at African-Americans and Hispanics tomorrow about how stupid they are for not voting for him? Or will he lead the crowd with a chant of 'Si Se Puede!'? Really who knows? It depends on who the last person it was he talked to, the feel of the crowd at the given moment.
The biggest liability Trump faces across the electorate is that he seems too mentally unstable to be president. He comes off as a charlatan. It is true that certain segments of the white community of scared off by his too open, too vehement racism. But with this latest twist he's done little to convince anyone that he's less racist or more moderate. He's mainly confused people. On the more driving issues of temperament, values or emotional grounding to be president this bizarre about face (which might about face again next week) just confirms the worst impression of the man.
I've heard a number of people say they don't believe any change of position or policy betrayal will shake his supporters. By and large I think that's right. The connection is emotive and thematic, not tied to any specific policy. But Trump's supporters in that sense of the word don't make up enough of the electorate to avoid a historic blowout. Even for them, some sense of being his mark rather than having him as their avenger can't help but blunt some of their enthusiasm. In any case, for the voters Trump needs it won't work.

This last point is crucial to understanding who Donald Trump is as a candidate for President. He has run a "emotive and thematic" campaign. The emotions are rage and fear. The themes are entitled victimhood and vengeance. His campaign has been almost policy-free as Trump has remained vague about specifics and done a lot of arm-waving about how be will "make America great again."
The lone exception to that has been immigration where he has been very specific. He proposed building a wall on our southern border and forcing Mexico to pay for it. He has also stated unequivocally that he will forcibly deport illegal immigrants and deny Muslim immigrants.
As his poll numbers have eroded, he has begun to waffle on deporting illegals and denying Muslims. Predictably, his hard-core supporters who pay attention to policy (e.g. Ann Coulter and Sarah Palin) have opposed his softer positions. White supremacist elements have insightfully said Trump is simply playing politics. Marshall is right that this latest public flailing is simply Trump revealing himself as a self-promoting charlatan.