Donald Trump holds a dominant position in national polls in no small part because he is extremely strong among people on the periphery of the Republican coalition.

He is strongest among Republicans who are less affluent, less educated and less likely to turn out to vote. His very best voters are self-identified Republicans who nonetheless are registered as Democrats. It’s a coalition that’s concentrated in the South, Appalachia and the industrial North, according to data provided to The Upshot by Civis Analytics, a Democratic data firm.

Mr. Trump’s huge advantage among these groups poses a challenge for his campaign, because it may not have the turnout operation necessary to mobilize irregular voters.

But it is just as big a challenge for the Republican Party, which has maintained its competitiveness in spite of losses among nonwhite and young voters by adding older and white voters, many from the South. These gains have helped the party retake the House, the Senate and many state governments. But these same voters may now be making it harder for the party to broaden its appeal to nonwhite and younger people — perhaps even by helping to nominate Mr. Trump.

Read the article “Donald Trump’s Strongest Supporters: A Certain Kind of Democrat” on nytimes.com.