The granddaddy of all U.S. stock indexes is about to notch its sixth-straight winning quarter, which will mark the best run since a six-win in the fourth quarter of 2006.

Among the mean feats the Dow industrials DJIA, -0.87% has managed for the first quarter of 2017 is to not only scale the 20,000 level for the first time ever, but to stay pretty solidly encamped there, brushing aside U.S. politics and even a Federal Reserve interest rate increase. And the index flicked at 21,000 in a handful of sessions at the start of March.