Michaels, I read the column and it was important. You are right correlation is not causation. One reason for that is called the ecological fallacy*. https://en.wikipedia.org/wiki/Ecological_fallacy That is because the people in the Red States may have been so particularly disadvantaged or advantaged by conditions. What you have to do is compare each poor person as an individual to see if that person is more likely to kill themselves than other individuals. And even then correlation is not causation.. But it surely cannot be done when you are comparing different levels of analysis.
But what Mr. Krugman posited was that this phenomena may well be the product of poverty that seems to be going up more in red states than in blue states. We can test it. You see correlation cannot prove causation, but it can suggest it. And with better analysis we can reasonably hypothesize that it does.
There is little doubt that poverty is a factor in suicide, but it is one of a constellation of variables that affects suicide. How much, I do not know. But Krugman's assertion may be true: support for the president may lead to more poverty which in turn may lead to more suicide.
*An ecological fallacy (or ecological inference fallacy)[1] is a formal fallacy in the interpretation of statistical data that occurs when inferences about the nature of individuals are deduced from inferences about the group to which those individuals belong. 'Ecological fallacy' is a term that is sometimes used to describe the fallacy of division, which is not a statistical fallacy. The four common statistical ecological fallacies are: confusion between ecological correlations and individual correlations, confusion between group average and total average, Simpson's paradox, and confusion between higher average and higher likelihood.