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Senator Bernie Sanders, the liberal Vermont senator who has been rising in recent polls in the race for the Democratic presidential nomination, raised $15 million in the last two months, his aides said.

The sum represents a third of what Hillary Rodham Clinton’s campaign reported raising by going after high-dollar donors. Instead, Mr. Sanders appealed largely to grassroots donors who could give smaller amounts to finance his candidacy.

The figure is surprisingly large for a fund-raising effort that has been primarily from small donors. And it will allow Mr. Sanders to air ads introducing himself to voters, as well hire staff that could help him maintain his candidacy.

Aides to Mr. Sanders said that since April 30, nearly 400,000 contributions had come into the campaign, with 99 percent of those donations in increments of $250 or less.

The disclosure of his fund-raising prowess comes as he has shown continued strength in polls in early nominating states, getting the support of 33 percent of likely caucusgoers in Iowa in one new poll and narrowing what was once a wide gap with Mrs. Clinton, who had 52 percent support.

On Wednesday evening, Mr. Sanders drew a crowd of roughly 10,000 at a speech in Madison, Wis., a Democratic stronghold.