AMHERST, N.H. — The Independence Day parades across New Hampshire on Saturday turned into parades of presidential candidates, and voters like Stuart Harmon were left shaking their heads at the daunting reality of watching, hearing and judging more than 20 contenders for the White House over the next seven months.

“This race feels more like a spectator sport than an election,” said Mr. Harmon, an independent who shook hands with former Gov. Jeb Bush of Florida and watched a dozen other campaign contingents march through this town of about 11,000. “Having all these candidates is a real problem, because it’ll be hard to hear each of them. I’m interested in Jeb Bush, but it’s not easy to pay attention to just one when there are so many.”

The likely field of 16 Republican candidates is stirring frustration, particularly among voters who say they feel more overwhelmed, even ambivalent, than ever before about their long-cherished responsibilities in holding the nation’s first primary. Some voters said they were already dreading the weeks of political fliers stuffed in their mailboxes, of campaign volunteers at their doors during the day and of television ads and automated phone calls all through the night. Others said they already had candidate fatigue.