Tell that to Carter and HW, if you fail to perform then incumbency is a huge burden and given Trump's deliverable promises, no one is going to believe his BS the second time around especially Dem voters who gave him a chance.
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).