Krugman is a very intelligent guy.
However, economics is not a science. It's an art. He's not an artist. He's a number cruncher who makes projections of the future using certain parameters.
Granted, very complex projections.
If the variables change the projection is no good.
Like most economists, he has also chosen a political side to back. Being from a liberal institution like Princeton, or because he is in fact a liberal, he has decided to back the left.
Good career move. If the Dems win in 2020 he'll be back on top, get invited to all the cocktail parties, where people will listen in awe to his pontificating.