But they’re not typical Republican primary voters, either — they may not even consider themselves Republicans. Mr. Trump fares best among people who identify as Republicans but nonetheless remain registered Democrats or have a history of voting in Democratic primaries — a legacy of their previous political allegiances — according to data from Civis Analytics, a Democratic firm.

One result is that Mr. Trump’s strength mirrors that of the Democrats in the middle part of the last century. It may seem odd to see Massachusetts paired with Mississippi as the top two states for Mr. Trump, but it’s something the Democrats pulled off quite regularly from 1928 until the passage of the Civil Rights Act.

Take Massachusetts, where Catholics made up a majority of the Republican electorate and provided Mr. Trump with a big primary victory. He drew 53 percent of Catholics in the Massachusetts G.O.P. primary, while Mr. Kasich and Mr. Rubio combined for just 35 percent. The story was the opposite among mainline Protestants — the traditional Republicans — who supported Mr. Kasich and Mr. Rubio over Mr. Trump.

There’s more to Mr. Trump’s advantage in the blue states than the new Republicans. His main opponent, Ted Cruz, fares best among voters of the religious right who identify in polls as “very conservative,” which makes him a bad fit for the more moderate blue states.

But Mr. Trump’s weak opposition is in part a product of his own strength in the blue states. It helped block the emergence of a mainstream candidate, like Jeb Bush, leaving conservative states to elevate Mr. Cruz, a candidate with little appeal to the blue states, as Mr. Trump’s principal rival.

The less religious Republican voters in the Northeast might be a factor in Mr. Trump’s strength as well. Mr. Trump does worst in areas with high church attendance, like Western Michigan or Utah. When he draws voters who are both working class and less religious — as in Massachusetts — he can really run up the score.

Another important factor is race. So far, there has been a strong relationship between Mr. Trump’s share of the vote by state and measures of racial animosity or bias. While no one suggests that all of Mr. Trump’s supporters are racist, surveys show that they are particularly likely to express explicit racial prejudice. And the Northeastern states often sit alongside the South at the top of these indicators, despite the Northeast’s reputation as a bastion of liberalism.