Clinton raised an additional $5 million for the Democratic National Committee and state parties during January, but that money cannot be used during what has become an unexpectedly close and contentious primary. Her campaign claimed it was the "best grassroots fundraising month of the campaign with 95 percent of the donations coming in increments of $100 or less."

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Although she remains the favorite to win the nomination eventually, the fundraising deficit in January is indicative of Sanders's momentum and grass-roots appeal. Nearly all of Sanders's donations have come from small donors, while Clinton's financial model is built on the traditional pyramid of wealthy donors and networkers giving the maximum $2,700 per person during the primary period.

"We invested early in organizing and that investment has already paid off with a Hillary Clinton victory in Iowa," Clinton campaign manager Robby Mook said in a statement. "Now, thanks to the support of more than 670,000 people across the country, we have the resources we need to take the fight to New Hampshire, Nevada, South Carolina and beyond."

Nevada and South Carolina hold presidential selection contests later in February, ahead of the crush of state contests in March. Clinton, long the national front-runner, had once been expected to have the nomination easily sewn up in February. Sanders, running on a groundswell of populist angst, battled Clinton to a near-tie in the Iowa caucuses on Monday and is likely to handily defeat her here. Clinton's campaign chalks her deficit in New Hampshire up to a hometown advantage -- Sanders is an Independent senator from neighboring Vermont.

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At the same time, the Clinton campaign has been sending increasingly alarmed email fundraising appeals, warning that Sanders was buying more television advertising than Clinton and that his supporters were coming through with donations at a furious clip.