Actually your reference to Carter brings to light the biggest factor for Presidents achieving a second term. The ECONOMY. In reality the economy has minimal connection to the president's performance but it the biggest factor nationwide in getting re-elected. Carter was running at a time when interest rates were 17%, inflation was out-of-control, gas prices were high, mortgages rates were setting records, job losses were wide-spread, and families had no confidence. Democrats can hope that an economic slump takes down Trump in 2020, but the indicators at this point don't make it seem likely (but 3.5 years is a long time ahead).
This is not about hoping - Trump promised jobs in sectors of the economy that are not going to come back and wages will stay stagnant, there is not going to be any great changes that he can campaign on, people will remember that things are exactly where Obama left them while highlighting that all the big promises of bringing jobs back was a lie. Even Carrier employees have realized that within a few months and that will be used as an example
Let's look again again at the 2016 voting results by county. Trump's "39% popularity" leads to results that look like this.
Agreed.Many people took a chance on Trump solely because he promised to bring to bring back manufacturing,coal,reverse trade deficits etc.A lot of those people will not be voting for him again.
You forgot to mention Hillary's unpopularity,the Comey effect along with the fact that many Dems gave Trump a chance based on his rhetoric - none of that will be in play in 2020. Dems will be motivated in 2020 unlike 2016 where many expected Hillary to win regardless of their vote, this time they have a target in Trump given how he has behaved in the WH.
It is more of a question is if your personal family is hurting economically or not. As a voter most individuals could care less about Trump's promises of coal jobs in West Virginia - I only care about my personal economic issues. If I lived in West Virginia coal mining country I might be quite upset with Trump failing to produce mining jobs. Living in the RTP area of North Carolina I only really care how the tech and pharma sectors are doing.
If they were hurting in 2016, nothing in Trump's agenda will stop the hurt in 2020 - it's only going to make things worse (see his healthcare bill). Republican agenda has never been good for the middle class voters and now Republicans can't deflect that away by blaming Democrats for not doing anything (after filibustering anything Dems do put forward)