Donald Trump’s win in New Hampshire’s Republican primary on January 23, a week after his decisive victory in the Iowa caucuses, means that he is almost certain to be the Republican nominee for the US presidential election in November 2024. All US presidential elections are different, but a renewed contest between Joe Biden and Trump is rather unusual.

It is rare for the same candidates to be nominated by their parties to run on two separate occasions. The last time it happened was in 1956, when the Republican president, Dwight Eisenhower, defeated Democrat Adlai Stevenson for the second time in a row.

American politics is currently very polarised in a way that was not true in the past. One source of this division is political ideology, which research shows plays a central role in defining people’s political identities.

A large proportion of Americans identify themselves as liberals or conservatives. People look favourably on those who share their ideological views, regarding them as “insiders”, while at the same time looking unfavourably on “outsiders” in the opposite camp.

The 2020 and 2024 elections have a lot in common, so we can use data from the 2020 American National Election Study to explore the likely role ideology will play in the contest in November.

The 2020 study surveyed more than 8,000 Americans both........

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