He is rabble-rousing elements of the broad, discontented, white working class and anxious "middle" classes which the establishment power system seeks to keep passive as the system of corporate oligarchy slowly and relentlessly grinds them down economically. In classic fascist/reactionary fashion, he diverts attention from understanding the actual structure and nature of society and instead scapegoats "outsiders," in his case "illegal" immigrants and would-be refugees seeking asylum from war zones. He has also identified the various trade treaties as a cause of the economic squeeze on workers, which is valid as a factor.
Trump has made a frontal assault on these so-called "free trade" deals, violating an establishment ideological taboo observed by Democrats and other Republicans alike. Trump has done more to incite opposition to these treaties than labor unions, which supposedly represent the interests of workers, and certainly more than the Democratic Party, whose dominant figures largely support these corporate-power-enhancing treaties.
And while pressing the same hot button "culture war" issues that "respectable" "conservatives" do, being a political vulgarian he doesn't do it in a muted fashion as standard-issue reactionary Republicans do but does it very sharply, as was on display in the third and last "presidential debate" last night between him and Hillary Clinton, the Democrat. (This sort of crudeness is referred to by Republican operatives themselves as tossing "red meat" to "the base." Apparently GOP voters are like pet ocelots. Trump threatens to take them off their leashes.) These issues are guns, abortion, and immigrants. (The immigrant issue of course is tied to the economic insecurity felt by millions, with racism and xenophobia in the mix.)
Trump laid down his marker on immigrants early on when he branded virtually all Mexican immigrants as rapists and murderers- a trope others have picked up, some implying that all murders in the U.S. are the work of illegal immigrants, an absurdity that goes unchallenged. Instead we get from Democrats and "liberals" heartstring-plucking stories of hardworking immigrant families (it always has to be families) and children caught in a legal bind not of their own making. Which is fine, but insufficient, as it leaves the murderer-immigrant theme unrefuted.
Trump lately has also been destabilizing the legitimacy of U.S. elections by claiming they are "rigged," to cheat him of his victory. He continued this tact last night. This is very cynical on his part, as he is doing it because he is apparently going to lose. 
But the irony is, U.S. elections ARE corrupt, only not in the ways Trump means.  For example, the last two times a Republican "won" the presidency, it was by stealing it. Bush the Younger stole Florida in 2000, aided and abetted by his brother Jeb, the then-governor of Florida, Jeb's GOP (Gang Of Plunderers) minions there who disenfranchised thousands of eligible voters, and most importantly by five Republican Supreme Court justices, who illegally blocked a recount in Florida. In 2004 Ohio was stolen, with the connivance of GOP officials there, led by Ohio Secretary of State Ken Blackwell. Since 2000, the Republican-owned corporations that make the crooked voting machines have connived to steal numerous elections by programming them to switch votes from Democrats to Republicans. (The Democrats refuse to make an issue out of any of this. As for 2000, they blame it all on Ralph Nader. Talk about scapegoating!)
But the Republicans (or elements of the secret police state acting on their behalf) are more ruthless than that.
Both Paul Wellstone and Mel Carnahan were murdered by plane sabotage to help the GOP control the U.S. Senate. And a devout Christian computer programmer, who helped the Republicans steal elections, had an attack of conscience and spoke out. While flying to testify in a civil case against Karl Rove, the top Republican electoral operative, his plane too was downed, and the evidence he was carrying to the trial was removed from the crash site, never to be seen again.
We can add mass disenfranchisement by various chicanery of Democratic voters, which the Democratic Party barely objected to until very recently.
Nor does it help that the Republicans who constituted a majority on the Supreme Court before the death of arch-reactionary Antonin Scalia last year gutted the Voting Rights Act, opening the door to even more ruthless disenfranchisement of "black" citizens in the erstwhile Confederacy.
Trump has even damaged the Republican Party, which he has hijacked for his ego trip. Contrary to universal handwringing by the establishment blatherariat, this is a good thing. It is to be fervently desired to see that evil, noxious weed break up into two or more parties. (That would also have the salutary effect of weakening the extortionist power of the Democratic Party to force people to vote for them as "the lesser of two evils" to avoid a Republican victory.)
You would think that progressives would welcome things that weakens the stability of the current oppressive, imperialist, and incorrigible U.S. structure. That assumes that they understand the problems they complain about are structural in nature and deeply rooted in the nature of U.S. society. Many apparently don't understand that, but they should.
But instead, as happens every time there is the one-day-every-two-years when some actual democracy has to be allowed- democracy to the very limited extent that some people (not millions of disenfranchised people, mostly blacks) are allowed to vote from a menu of two choices presented to them- Democratic Party politicians or Republican Party politicians- what progressives and special interest "leftists"* virtually all do is line up like obedient ducklings behind the Democrats.  Because "the Republicans are worse." They are exhorted by Democratic Party political auxiliaries like Robert Reich (a Bill Clinton cabinet member) that "now is not the time" to seek an alternative, not even a Senator Bernard Sanders, who ran as a Democrat for president against Hillary Clinton in the Democratic Party primaries and then supported her when he was blocked from victory. But there will never be a "right time" as long as the Republican Party exists.
Preserving the faux "unity" of the American populace under the current political and economic power structure is precisely what those in power want! Why would anyone who struggles against this system want to help do that?
The media created Donald Trump, making news stories out of even his random idiotic tweets. Now they are trying desperately to destroy his drive to the White House with sex scandalizing. To be sure, he is loathsome, and a probable rapist, as I will explain in a separate essay. (Bill Clinton is a confirmed rapist- cf. Juanita Broaddrick- and that IS relevant because he will have immense power if/when his wife achieve her goal of the presidency, her compensation for setting aside her own ambitions in Arkansas and when her husband got to be president. It's a family affair, you see. Now it's her turn. The other relevance is that Hillary was a driving force in the character assassinations of her husband's various paramours. And it went into actual terrorism. When Kathleen Willey reported a rebuffed sexual advance on her by Bill C. in the White House, her cat was killed, after which a man jogged up to her outside when she was along and mentioned the dead cat. A scary threat indeed, the implication being that he could kill her right then and there.)
Here's a good reason to look forward to the end of the election: the media obsession with Donald Trump will abate. (If we're very lucky, it will even end.) But the barbaric soul of the Republican Party has had its genteel veil torn off by Donald Trump.
1] Trump also has broken new ground, at least in the past century, by openly attacking the legitimacy of his two-party opponent. Trump at the previous debate said he would jail Hillary Clinton if elected. Last night he said she shouldn't even be allowed to run because of her "many crimes." If he had meant crimes against humanity, he would have been onto something. He just meant her private email server, destroying 30,000 emails, an allegedly lying to the FBI.
This attack on the legitimacy of the official opposition can be seen as rooted in the GOP attacks on Obama's legitimacy, including the claims that he was born in Kenya and thus Constitutionally ineligible to be president. The GOP didn't overtly push that line, but its auxiliaries in "talk" radio promoted it. Trump took that "issue" and ran with it in the years when he was preparing the ground for his attempt to grab the presidency.
Nor are fraudulent claims of election-stealing by the Democrats new. The myth has long been nurtured that JFK stole the 1960 election from Nixon by stealing Illinois. That myth is easily refuted by simple arithmetic. Just subtract the Illinois electoral votes from Kennedy's total and add them to Nixon's. Kennedy still wins. But as Reagan said, "facts are stupid things."
In fact, the Republican Party, by pushing relentlessly to the right (with the help of almost the entire U.S. media) over the last almost 50 years, a campaign of reaction and rollback that was a response to the upsurges of the 1960s, has really opened the door to a demagogue like Trump. The years of rightwing extremism have validated the derangement and inculcated the rightwing segment of the populace in irrational beliefs an animosities.
2] Trump claims illegal immigrants are voting, "dead people" are voting, illegal immigrants are not just voting once, but "fifteen times," etc. etc. This is of a piece with his general cynical demagoguery designed to rabble-rouse the racists and xenophobes, who number in the millions.
3] I use "leftists" for want of a better single word, and also because that is how the power structure thinks of them, especially the secret police agencies, "talk" radio, hard-rightwing media, and rightists generally, even though that is a stupid way to think of them. I am referring to single-issue people such as those categorized as environmentalists, gay rights activists, "civil" i.e. black rights people, and so on, dozens of such splinter groups with no unity, each applying a modicum of pressure on the system to achieve its own ends within the current system without changing its basic imperialist, corporate oligarchic nature one whit.
The truth is, the vast majority of progressives are frightened of pulling down the temple of power. That's why their distress at Trump's refusal to validate the fairness of the election has them in a tizzy as much as it has the establishment media the the Democrats. Even as I'm writing this, one Kristen Clarke is talking to Amy Goodman on Democracy Now! (the first segment of the day's show) and prattling on about U.S. "democracy" being the "model for the world" and how "distressing" it is that Trump would question the legitimacy of a U,S. election. Then she segued into a total non sequitur about voter suppression against blacks! Which would seem to undercut her boilerplate propaganda about legitimacy and a Model for the World. She keeps referring to "conspiracy theories," and brushes aside a question about Gore v. Bush in 2000 ("We're not going to relitigate [that]" i.e. not going to discuss, remember, or learn from) and denies U.S. elections can possibly be rigged. "Rigged elections, pure mythology, pure fiction..."! [What, 2000 wasn't? 2004 wasn't?]
Clarke is also bent out of shape that Obama's nominee for the "broken" Supreme Court, Merrick Garland, a conservative former prosecutor, hasn't been given a Senate confirmation hearing. "A Constitutional Crisis" she calls it. "I am deeply concerned about this era of political obstruction we are in," she says, parroting Democratic Party rhetoric.
Clarke is President and Executive Director of the "Lawyers' Committee for Civil Rights Under Law," founded in 1963 at the request of president John F. Kennedy. That political pedigree maybe tells you all you need to know about where Clarke is coming from.