While many of Mr. Trump’s fans are committed, others weren’t ready to pledge to a candidate or party, for that matter. “I’m looking at Trump and I’m looking at Hillary. That’s a difference,” said Petrina Sullivan, a registered Democrat from Hudson, N.H., who attended the Lowell rally. “There is something about him that I find very appealing.”

New Hampshire allows undeclared voters to choose their party on Election Day, creating a high degree of fluidity among the electorate. The open process has traditionally driven high turnout, but a new law requiring voters to show government-issued identification or filling out paperwork at precincts before casting their ballots could limit participation, political observers said.

State exit polls show that the number of voters who pick a presidential nominee in the final days of the campaign has grown. In 2012, 46% of GOP primary voters chose their candidate in the three days leading up to the election, up from 39% in 2008 and 26% in 2000, University of New Hampshire Survey Center data show.

“You shouldn’t believe in polls in New Hampshire until the weekend before the election, and I’ll tell you that as a pollster,” said Zachary S. Azem, a survey center researcher.