"Here's the truth: We know we have more supporters than he does. But a higher proportion of his supporters have stepped up to fund his campaign," campaign manager Robby Mook wrote in an email to Clinton supporters with the subject line "This is what being outraised looks like."

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The email said Sanders's campaign is outspending them on TV in Iowa and New Hampshire "just as voters are starting to pay attention, and the polls are incredibly tight."

It asked for supporters to give funds — just $1 — to "help close the gap."

"I'm excited to fight by your side to make college more affordable, to provide health care and raise wages for American families, to take on the gun lobby and make our communities safer, and so much more," the email continued, adding that first, Clinton needs to win the nomination.

The former secretary of State's campaign has been trying to fundraise as Sanders surges in polling in the early-voting states of Iowa and New Hampshire.

In an email sent Saturday, the campaign asked for supporters to chip in, saying that the polls have shown "we have a real race on our hands."