In the wake of last November’s election, many Democrats blamed low turnout for Kamala Harris’s defeat.

It wasn’t entirely without reason, as turnout dropped in Democratic areas, but many months later it is clear the blame was misplaced. Newly available data, based on authoritative voter turnout records, suggests that if anything, President Trump would have done even better if everyone had voted.

The new data, including a new study from Pew Research released Thursday, instead offers a more dispiriting explanation for Democrats: Young, nonwhite and irregular voters defected by the millions to Mr. Trump, costing Ms. Harris both the Electoral College and the popular vote.

The findings suggest that Mr. Trump’s brand of conservative populism once again turned politics-as-usual upside down, as his gains among disengaged voters deprived Democrats of their traditional advantage with this group, who are disproportionately young and nonwhite.

For a generation, the assumption that Democrats benefit from high turnout has underpinned the hopes and machinations of both parties, from Republican support for restrictive voting laws to Democratic hopes of mobilizing a new progressive coalition of young and nonwhite voters. It’s not clear whether Democrats will struggle with irregular voters in the future, but the data nonetheless essentially ends the debate about whether Ms. Harris lost because she alienated swing voters or because she failed to energize her base. In the end, Democrats alienated voters whose longtime support they might have taken for granted.

The 2024 election may feel like old news, especially in the wake of Zohran Mamdani’s upset victory in New York City on Tuesday, but the best data on the outcome has only recently become available. Over the last two months, the last few states updated their official records of who did or did not vote in the election. These records unlock the most authoritative studies of the electorate, which link voter turnout records to high-quality surveys.