WASHINGTON -- Donald Trump will likely become his party's presidential nominee if he wins Indiana on Tuesday, capping a string of dominance across vast swaths of the United States.

There's one glaring exception.

Just look at the map.

The middle of the country has rebuffed Trump. He's gone zero for 10 in Republican contests there. He's lost mainly to Ted Cruz, but also to Marco Rubio. He's lost primaries, caucuses, and delegate conventions.

What political kryptonite lies in America's heartland, rendering Trump so feeble there? The question carries relevance Tuesday: Indiana bills itself as the Crossroads of America, and sits near the meeting point of the pro- and anti-Trump regions.

"This is clearly a multi-faceted story," said David Byler, a researcher at the site Real Clear Politics.

Political analysts, activists, and data offered some clues.

Race and identity

There's a racial difference.

White voters massively preferred Trump in states with a heavy minority presence (Alabama, Mississippi, New York, Nevada). In whiter states, they didn't (Idaho, Kansas, Oklahoma).

Census data compiled by The Canadian Press shows that states won by Trump averaged three times more African-Americans (16.3 per cent of the population) than states won by Cruz (5.3 per cent).

In states without minorities, the candidate who proposed making Mexico pay for a wall and getting tough with Black Lives Matter protesters had less success.

Political science has a well-documented term for this: the "racial threat hypothesis."

"Whites who live in close proximity to minority populations tend to be more open to racialized appeals," Byler said, explaining the concept.

"I want to be clear: obviously not all of Trump's appeal is racial -- a lot of it is economic, a lot of it is cultural. But some of it may have something to do with race."

Trump supporters are by far most likely to call immigrants a burden on the country; 69 per cent expressed that feeling according to Pew Research, compared to 51 per cent of Cruz's and 40 per cent of John Kasich's.

Activism

Trump fares best with political outsiders, Cruz with heavily active Republicans.

Several western contests played to Cruz's strength.

Four out of five Cruz wins have come in states with caucuses, which involve a public meeting. These include Iowa and Kansas. He also dominated in Wyoming and Colorado -- where the general public didn't vote, and delegates were elected at conventions.

There's just one caucus state in the east: Maine. It's the only one Cruz won in that region.

Ideology

Trump's supporters are sometimes conservative -- just like their candidate. Cruz's are conservative across the board.

Take the size of government: Cruz won states where Pew polls showed the highest support for cutting government, and services.

On abortion, Trump voters mildly skew pro-life. It's no contest among Cruz's -- according to Pew, 73 per cent say abortion should be illegal in all or most cases.

One party activist in Colorado said people don't trust the New York billionaire on gun rights and abortion.

Laura Carno called him a conservative of convenience: "He has spoken on the record far more in support of pro-choice policies over his career. They just don't believe him."

Religion

The most active churchgoers support Cruz. Trump does well with evangelicals generally, but loses to Cruz by 15 points nationally among voters who attend weekly service, Pew says.

Trump lost the Lutheran-heavy region in the Midwest. But he's especially toxic among Mormons, concentrated in Utah and surrounding states.

Trump finished third in Utah and lost by 54 points.

Resentment of elites

Trump supporters are likeliest to feel ignored. The Rand Corp. found a stunning statistic. It said the best way to identify a Trump supporter in its research was to ask: Do people like you have any say about what the government does?

Those who answered, "No," were 86.5 per cent more likely to prefer Trump.

A gap appears when it comes to personal wealth and education.

Among Republicans making under $30,000, Trump was trouncing his rivals by 20 points according to Pew -- while running neck-and-neck with those earning over $100,000. He did best with high-school grads; competed among Republican college grads; and lost with postgrads.

That can't be explained by regional differences, one political analyst said.

Kyle Kondik said Trump's been so dominant he wouldn't read much into a few smaller-state results.

Anyway, it might not matter much after Tuesday.

Said Kondik, managing editor of Sabato's Crystal Ball: "I think it might be all over but the shoutin' if Trump wins (Indiana)."