His involvement in politics is largely limited to the past several years. His main contributions to economics, arguably, have come far before that.
He's the author of the foremost textbook on international trade (Krugman+Obstfield), and developed models of international trade that every economics major knows (or should, at least).
(1) Krugman won the John Bates Clark Medal for economics in 1991. This is a Prize given every two years to an American Economist under 40. It's often considered harder to win than the economics Nobel, and is a significant predictor of who actually will win the Nobel. Note that this was largely before Krugman got involved in politics, and past winners include Milton Friedman. I doubt his politics helped him win this one.
(2) By most measures of citation impact, Krugman is one of the world's leading economists. E.g., http://ideas.repec.org/top/top.person.nbcites.html
Citations are, of course, an imperfect predictor of the significance of someone's work, this is just an additional data point.
I doubt his involvement in US politics had much impact on the prize.