WASHINGTON — In 2012, B. Rowe Winecoff, a retired social worker from Newton, Iowa, contributed $120 to President Obama’s re-election campaign. But he has yet to give any money to Hillary Clinton in this election. “This year just seems dirtier than ever,” said Mr. Winecoff, a Democrat, explaining why he has not contributed to the candidate he intends to vote for.

Even as newly released financial disclosures reveal that Mrs. Clinton enjoys a substantial fund-raising advantage over Donald J. Trump, she is struggling to replicate the sort of small-dollar juggernaut that Mr. Obama enjoyed in his campaigns and Senator Bernie Sanders relied on in this year’s Democratic primaries.

In an illustration of the lack of enthusiasm for her among some liberal activists, just 24 percent of the contributors to Mrs. Clinton’s campaign so far have given $200 or less. In 2012, 43 percent of the money to Mr. Obama was from contributors who gave $200 or less, and this year 58 percent of the giving to Mr. Sanders’s grass-roots bid came from small-dollar donors.

Without this online network, Mrs. Clinton is being made to continue with an aggressive calendar of fund-raisers with rich donors as Election Day grows near — events that can limit her time in swing states and reinforce concerns that give rank-and-file Democrats pause.