One by one Trump alienates voters who held their noses and voted due to a particular promise, regardless of their overall feelings about him.  

The working-class Trump vowed to rescue from economic malaise now realise they are once again being set up to be fleeced in everything from retirement investments to housing as Trump deregulates the financial industry that brought them  the 2008 financial crisis.  California farmers, who might have voted Trump for a renewed military, now understand that they might as well not plant what they can't harvest - and without itinerant Mexican workers they can't harvest.  Their business has just been shifted to Mexico; California will now import produce. Those who wanted to drain the swamp now realise Trump has just re-stocked the swamp with his billionaire pals - new and much worse alligators.

As each particular group is let down or attacked, Trump's approval is diminished that much more.  The  mid-terms will end his political power.  2 years left!