You're correct that people got things wrong before but I think your conclusion is off base. First off, people mostly doubted Trump would get the nomination early on in the process, before he acrually startes winning. Once he started winning they mostly treated hin like the front runner he was. Second, the people who predicted that Trump couldnt win the presidency were basically right. Trump lost the popular vote dramatically (in fact he got less of the popular vote than any losing candidate since 1996). Which brings me to your conclusion. Your argument is basically that people keep underestimating Trump but that he outsmartd them and comes out on top. But I dont think that a reading of the 2016 election bears that out. Trump didnt win so much because he was a smart candidate. He ran an awful campaign in many ways. Instead, he won because his opponent had a truly horrible piece of bad luck right before the election. The states which put Trump over the top in the electoral college were all won by 0.7% of the vote or less. At the end of October, Clinton was sitting on a 5-6 point lead in national polling. Without the announcement from Comey that he was looking into her emails again she would have trounced Trump. The other factor which helped Trump was shear party loyalty. Something like 80% of his voters said that they were voting against Clinton rather than for him. Many had grave doubts about Trump simply weren't willing to abandon the party.
Here's why Trump is unlikely to win in 2020. I'm not basing this off just hope, or off of a vague notion that people underestimated Trump or not. I'm basing this on the fundamentals of trump and the 2020 election. Of course, lots of things could change between them and now that could help Trump's chances. But, as it stands now, this is my analysis. What we start with is the fact that, as I mentioned before, Trump lost popular vote. You lost it by pretty decent margin to almost 3%. That gives him the third lowest margin of victory any American president ever. So the bottom line is that Trump failed at the basic job of a campaign, getting people to vote for him. Trump was an unpopular guy in the 2016 election. In fact he was the least popular presidential candidate, in terms of approval ratings, since they began doing polling. Unfortunately for Trump, his numbers only got worse as he's become president. Currently, he's usually performing in the low to high thirties. That's a level of unpopularity that Barack Obama never reached and which most presidents only reach in the depths of major scandal. My point is that Trump shedding support. The number Trump voters who had doubts about him but who voted for him anyway thinking that they would at least give him a shot the problem is that more of these voters, especially among independents, have given him a shot and are turning against him. Trumpet certainly turn this around in the next few years, but it's a big problem for him.
As I mentioned above, Trump very narrowly won the 2016 election. He lost the popular vote, but he's Electro Victory wasn't that great either. He had the third lowest electoral vote total in the last hundred years. And he only got that vote total by winning several states by the narrowest of margins. The thing is, trumpet Chief this incredibly now Victory while running against the least popular Democrat ever nominated. Hillary Clinton was the second least popular candidate since they began modern polling. Should someone who is known as a hard worker, but a poor campaigner. And yet Donald Trump lost the popular vote and almost lost the Electoral College to her. If you could only barely win against Hillary Clinton, what is he going to do in 2020 when he's running against Democrat who almost certainly will be at least a little bit more popular than her and will have better political instincts.
The other problem for Trump is that the demographics of 2020 will be against him. There's an anecdote that I like to mention about the problem of trump faces. In 1984 Ronald Reagan got over 60% of the White vote. This was enough for him to decisively win the popular vote and to get over 500 electoral votes. Donald Trump also got about 60% of the white vote. But he lost the popular vote and only got 300 electoral votes. The difference is that in the intervening 32 years the white population of the United States has shrunk as a percentage of the whole. It's going to decline even further by the time Trump seeks reelection in 2020. That's going to be a major problem for Trump, because almost all of his support comes from white people. Many states are also getting less white. Hillary Clinton, for example, one the largest victory in California in a century. She did even better than California natives like Ronald Reagan and Richard Nixon. That's in part because California is no longer a majority white state. Another example is Virginia, would used to be reliably Republican state, but which Democrats have won three times in a row. The change is prompted largely by the growth in the Black and Hispanic population of Virginia North Carolina seen a similar change although it is still competitive for republicans. Many states around the country, particular States, or seen increases in their black, Latino, and Asian populations. If you look at Texas, you can see some of this. In 2016, Donald Trump did nine points worse in Texas than Mitt Romney did four years earlier. The other demographic trend which will work against Trump is age. The only age group that Trump won decisively was his own, the Baby Boomers. He and Clinton were more evenly split among middle-aged voters, but he decisively lost among voters under 30, the so-called millennials. The problem is that everyday Baby Boomers die off and every day more Millennials turn 18 and become eligible to vote. Millennials are already the largest age group in the electorate and that will only increase in the next four years. This will make it more difficult for Trump to win. Remember that in the critical states which gave him an electoral college victory, he only won by less than 1%. It seems entirely plausible that the 2020 electorate, which will be less white unless dominated by the Baby Boomers, will not deliver those narrow victories to Trump.
Finally, I don't want to act like Trump's victory was entirely a fluke. He certainly had a message which you push strongly on voters and which appealed strongly to a significant minority of voters. The problem for Trump is that this message will likely be far less potent in 2020 then it was in 2016. Trump ran in 2016 as an outsider candidate. He promised to shake up the system and bring business management expertise to Washington. On policy, he made lots of extravagant claims about how good things were going to be under his leadership. This is only capsulated in his slogan make America great again, one of the few truly brilliant pieces to his campaign. But most of these arguments and themes are either not going to be available Tim to in 2020 or will not be as effective. Trump can't run as an outsider who's going to shake up the system when he spent four years as a part of that system. Personally, I think the actions that he's taken such as pointing lots of Wall Street Banker types to positions of power, will also undermine his claims to be looking out for the little guy. Ears, he want some have a record of accomplishments when she can run on. That might be a positive, if he's accomplished a lot, but it also means that he won't be able to make the kind of extravagant promises which he did in 2016 and which helped attract at these some people to his cause. Trump also risks alienating some of his supporters if he doesn't end up fulfilling promises when she made such as building the wall, restricting immigration, and bring an American manufacturing roaring back to life. One example of this is coal. Trump portrayed himself on the stump as a great friend of the Coal Miner's who would bring back their jobs by attacking environmental restrictions. But coal suffering mainly from market competition and not regulation. Will voters in Coal country still love Trump when it turns out that he can't save their jobs. In some ways, Trump benefited in 2016 from being a blank slate that voters could put whatever hopes they wanted onto. Button 20/20 he won't have that advantage. You will have to contend with what he's actually done good and bad. In 2016, people could wish away many of the possible downsides a trump presidency. In 2020 it will be much harder to do that because we will have the evidence of his presidency in front of us. People will not be able to as easily ignore things like his vulgarity, his vindictiveness, his immaturity, is nepotism, his corruption, his ignorance of issues, and just general poor management skills. Hillary Clinton tried to make issues of these during the 2016 campaign but Trump was able to convince enough voters that these were not going to be a problem. I think that he will have a much harder time doing that in 2020.
For all these reasons of fundamentals I think that Trump will likely lose in 2020.