Democrats won the House as voters across nearly all demographic groups moved to the left, especially women and young people, according to exit polls.

White women swung left, but House Democrats failed to win the group outright.

In the 2016 presidential election, 55 percent of white women voted for Republicans. And this year, the group backed Democrats and Republicans evenly.

Historically, women, people of color and young voters have been more likely to cast ballots for Democrats, while men and wealthier voters have tended to favor Republicans. These demographic divisions held in 2018, but the last few decades of exit polls show that that has not always been the case.

Women broke hard for Democrats this year, even more so than usual.

The gender gap has remained relatively consistent since the 1980s, but it has been widening in recent years. Political scientists attribute this to women placing more of an emphasis on social welfare issues like health care and child care, which generally align with the Democratic Party, whereas men are more concerned with issues like taxes and national security.

The age divide among voters is a relatively new phenomenon.

There was not much of a partisan age gap until the mid-2000s, fueled in part by young Americans’ discontent with the aftermath of Sept. 11, 2001, and propelled by Barack Obama’s presidential candidacy, said Brian F. Schaffner, a political scientist at Tufts University. This year, voters under age 30 broke for Democrats by a 35-point margin.

All racial groups moved left, but white voters remain solidly Republican.

“Race is the most pronounced partisan difference in how people vote, especially among white and black voters,” said Dr. Schaffner. Latino voters, on the other hand, are a much less homogeneous group. This year, Asian voters swung left more than any other voters of color.

Middle-income voters returned to the Democratic Party.