Democratic presidential contender Hillary Rodham Clinton now has a donor base of more than 500,000 supporters, thanks in part to a rush of new contributors who gave money after her 11-hour congressional testimony about the Benghazi attacks Thursday.

Clinton officials said that her campaign had its best fundraising hour yet from 9 p.m. to 10 p.m. Thursday, after the former secretary of state wrapped up her testimony. Even though the campaign did not put out a request for contributions, small donations flooded in online — half from people who had never given her money before.

Clinton's pool of donors is still smaller than that of Sen. Bernie Sanders of Vermont, who has attracted more than 760,000 individual givers, his campaign said.

But she has been steadily expanding her base, which numbered around 250,000 by the end of June. By the end of September, Clinton has hit nearly 400,000 donors. More than 100,000 new people have come aboard already this month.

The large bases of small donors that she and Sanders have built are helping the Democrats bring in millions more than their GOP counterparts, many of whom are leaning more heavily on the support of big-money super PACs, which cannot directly coordinate with their campaigns.

John Wagner contributed to this report.

This post has been amended with an updated total for Sanders's donors.