I blame Reagan – for pretty much everything bad that’s happened in the US political system since his campaign.
He’s the guy who crystallized the idea “Government is the problem – not the solution.” That’s almost never true, but it gained an enormous following and led to the Tea Party and eventually straight to Donald Trump himself.
But to more directly answer the OP. Trump’s been unstoppable because he’s only had to face people as answer-free as he is.
So far. He’s gonna get stomped and hit in the head with a brick come November.
Reagan also pushed the idea over selling the candidate’s image over the substance of his policies; the idea that it’s more important to like the candidate personally than agree with him on political issues.
It’s like a zebra’s camouflage. It seems like black and white stripes would be terribly obvious, and lions would have no problem picking them off one by one. But, there are so many zebras with the same coloring, that lions can’t pick out a single zebra to sink their teeth into; so in a herd all of that obvious coloring is impossible to discern. Trump as a candidate has so much that is ridiculous about him, that no single one thing ends up sticking.
This sounds like Mr. Burns, who has EVERY disease there is, so no one of them can quite manage to kill him.
Becaise he so thoroughly encapsulates the modern Republican Party base.
Up until now he has only had to be the biggest fish in a small pond. The other fish were defective and deformed. None of the other Republicans took him seriously until it was too late, none of them called him out for his lies, none of them exposed him as the uneducated dunce that he is. On top of that, the Republican voters simply didn’t care if he knew anything, they have no use for facts, all they care about is the ability to project strength. They’d rather have someone be wrong at the top of his lungs than give a nuanced opinion. All of that has changed. Hillary isn’t afraid to call him out. Neither are Obama, Biden, Sanders, Bill Clinton, and Warren. Trump’s surrogates can’t defend him because he is indefensible. So the key words in the OP are “up to now”. But now is quite different than two months ago.
Because THIS scene from Family Guy, is sadly, closer to reality than one might think.
I addressed this a while ago. People like to characterize the Trump Republicans as tards but this is nonsense. The Trump supporters are not any more “stupid” than the average Democratic voter. If you picked 100 Trump supporters at random and 100 Hillary supporters at random who do you think would score higher as a group average on an SAT or Wonderlic test? You choose Hillary and I’ll bet I can take some money from you in a bet. A lot of Trump supporters are people running businesses, making payrolls, fixing and servicing complex shit that you might struggle to comprehend.
They KNOW what Trump is spouting is racist gargling, and they don’t care they like it because … wait for it… they are racists and they are ecstatic that they can let the mask drop a little. Racists are making the world go around just like non-racists. Thinking they are stupid may be comforting emotionally, but it is huge mistake strategically in defeating them.
What difference does it make whether you treat them as kings of the world or cockroaches (the latter being far closer to their actual condition)? They’re still going to vote for the worst candidate ever. Idiot, loony, anarchist, what’s in a name when they stink just as much?
He’s a politician? 
If you scrunch up your eyes just right…
“Never underestimate the power of stupid people in large groups.”-George Carlin
Reagan! I would have loved to have had a beer with him! And asked him why he dumped Jane Wyman to marry the Blow Job Queen of Hollywood.
Is this a trick question?