Mrs. Clinton’s campaign took in $84 million last month, which officials said was the best fund-raising month of the campaign. The figures leave Mrs. Clinton well positioned for a final spending splurge on television, digital outreach and field mobilization as polls show her moving slightly ahead of Donald J. Trump.

The figures show Mrs. Clinton behind the blazing pace set by President Obama in his 2012 re-election bid, when Mr. Obama and the Democrats raised $181 million in September.

In past months, Mrs. Clinton has performed less well with small donors than Mr. Obama and her former rival Senator Bernie Sanders of Vermont had.

The Clinton campaign did not release details that would provide insight into her small-donor fund-raising in September.