Sen. Bernie Sanders' presidential campaign says the Vermont Democrat has raised $15 million since joining the race in late April, a strong total for his challenge against front-runner Hillary Rodham Clinton.

About 250,000 donors have given money.

Sanders has drawn large crowds, including about 10,000 in Madison, Wisconsin, on Wednesday night, and the fundraising amount will bolster his attempt to become a liberal alternative to Clinton. But Sanders trails by a wide margin in fundraising, with Clinton's campaign saying it has taken in $45 million since mid-April.

"I am more than aware that my opponents will be able to outspend us," Sanders said in Wisconsin. "They may have the money but we have the people. And when the people stand together, we can win."

The Sanders campaign said it had received nearly 400,000 contributions for an average donation of $33.51, and most online donations were made through the campaign's website.

Taking into account all sources of campaign revenue, including the sale of T-shirts and other merchandise, nearly 87 percent of the money came from donations of $250 or less, the campaign said.

Sanders is courting the most liberal grassroots voters by running largely on a platform of reducing income inequality. His stock has risen in early states like New Hampshire and Iowa, but he still trails Clinton in early polling.

The 73-year-old democratic socialist has tried to appeal to Democrats with a message of raising the minimum wage to $15 an hour and increasing taxes on the wealthy and Wall Street.