Mrs. Clinton has a considerable lead over Mr. Trump among newly registered voters in Pennsylvania, Florida and North Carolina combined, 47 percent to 31 percent.

Her edge among newly registered voters has often been lost in analysis of voter registration trends that focus on changes in party registration since 2012. Newly registered voters increasingly avoid affiliating with a major party, so they don’t have a big effect on voter registration tallies: In the three states we analyzed, Democrats have a modest advantage among voters who registered since 2012, 34 percent to 28 percent, according to data from L2, a nonpartisan voter file vendor.

But the newly registered voters nonetheless solidly lean toward Mrs. Clinton, based on our polling data and voter records. They’re disproportionately young and nonwhite.

Newly registered voters who aren’t affiliated with a major party lean to Mrs. Clinton over Mr. Trump by 42 percent to 21 percent; Gary Johnson runs a close third, with 20 percent. They may not improve the Democratic registration edge in these states, but they could contribute to Mrs. Clinton’s margin on Election Day.