But Mr. Trump’s lead is not equal among all G.O.P. groups, or across all parts of the country. His support follows a clear geographic pattern. He fares best in a broad swath of the country stretching from the Gulf Coast, up the spine of the Appalachian Mountains, to upstate New York.

Mr. Trump’s best state is West Virginia, followed by New York. Eight of Mr. Trump’s 10 best congressional districts are in New York, including several on Long Island. North Carolina, Alabama, Mississippi, Tennessee, Louisiana and South Carolina follow.

His strength in the South is blunted only by Ted Cruz in Texas and Mike Huckabee in Arkansas. (Mr. Huckabee, despite his weakness nationally, still holds a lead in the congressional district of his Arkansas hometown.) Mr. Trump fares well in Florida despite the political histories of Jeb Bush and Marco Rubio in the state.

The margin of uncertainty around the congressional district estimates is plus or minus 8.7 percentage points, even after more than 11,000 interviews and the benefit of modeling. The data also reflects the preferences of Republican 2016 general election voters — a smaller group than all registered voters, but larger than a primary or caucus electorate. The broad pattern in the Civis data is still clear, however.

Mr. Trump’s strength fades as one heads west. Nearly all of his weakest states — 16 of his worst 19 — lie west of the Mississippi. Mr. Trump’s struggles in Iowa might not reflect a challenge specific to the state; it may simply be the only state from the Great Plains or Mountain West where public pollsters frequently conduct public opinion surveys. His worst is Utah, a traditionally Republican and affluent state.

His geographic pattern of support is not just about demographics — educational attainment, for example. It is not necessarily the typical pattern for a populist, either. In fact, it’s almost the exact opposite of Ross Perot’s support in 1992, which was strongest in the West and New England, and weakest in the South and industrial North.

But it is still a familiar pattern. It is similar to a map of the tendency toward racism by region, according to measures like the prevalence of Google searches for racial slurs and racist jokes, or scores on implicit association tests.