The following report is from PolitiFact, which has partnered with NBC News to fact-check candidates during the 2016 political season.

Following victories in Florida, North Carolina, Ohio and Illinois, Democratic presidential candidate Hillary Clinton thanked her supporters — especially those who have given small amounts of money to her campaign.

“Keep contributing at hillaryclinton.com. Please, please join the 950,000 supporters who already have contributed, most less than $100, because our campaign depends on small donations for the majority of our support,” Clinton said.

We wondered whether it’s accurate to say that her campaign “depends on small donations for the majority of our support.”

This is a surprisingly tricky question to answer.

Under Federal Election Commission rules, campaigns must register the personal data of donors who give $200 or more. (That includes name, occupation, employer and ZIP code.) That $200 amount can be in the form of one donation of at least $200, or in a series of donations that collectively equal at least $200.

However, if a donor gives an amount less than $200, the campaign is not required to disclose the donor’s name. Such donations are often called “small-dollar contributions” or “unitemized donations.”

Candidates like to play up their small-dollar donations because they are a sign of grass-roots strength. For instance, the Bernie Sanders campaign often talks up its large small-donor base and small average donations.

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And in fact, of all the 2016 candidates, Sanders has done the best in this regard: Through Jan. 31, Sanders raised $67 million from small donors, or a whopping 70 percent of all the money he took in from individuals, according to the Center for Responsive Politics, an independent group that tracks campaign finance data.

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Clinton, for her part, raised a little under $22 million from small donors over the same period, or 17 percent of her total fundraising haul from individuals. That percentage rose a bit with the release of donations through Feb. 29 — Clinton has now raised more than $28 million from small donors, or 19 percent of her individual donations. (Among the remaining Republicans, Ted Cruz had the highest small-donation percentage with 42 percent. Donald Trump had 22 percent and John Kasich had 11 percent, though both Trump and Kasich have taken in much smaller amounts overall from small donors.)

So, compared with Sanders, Clinton has raised a smaller dollar total from small donors, and a smaller percentage of her individual contributions have come from from small donors. In other words, if she wants to emphasize the strength of her grassroots support, she needs to find different statistics to highlight.

Clinton spokesman Josh Schwerin said that more than 50 percent of the money the campaign has raised in February and March has come from what they call “online grassroots donations.” This is a continuation of a pattern the campaign originally noted in a Washington Post article in mid February.

And when the campaign released its campaign-finance data through Feb. 29, it reported that the average donation in February was about $50, and that 98 percent of donations were smaller than $250.

These do sound like strong statistics to bolster the campaign’s grassroots cred — but it also highlights a mismatch between what the campaign says and what fact-checkers can verify.

Put simply, once the debate turns to the number of donors and the scope of online donations, reporters simply have to trust that the campaigns are being honest.

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Consider the number Clinton cited — that she has 950,000 individual contributors. (That’s about what the Sanders campaign said he had in January 2016, when it said it had just passed the 1 million donor mark.)

Figures such as these come from the campaigns’ internal databases and are not filtered through the federal disclosure system, where they can be analyzed by independent groups. Indeed, the statistic Schwerin cited hasn’t even been submitted to the Federal Election Commission yet. And no campaign has thus far let fact-checkers dig into their donor lists.

So just knowing the amount of money raised from small donors won’t tell you how many people gave.

“A lump-sum unitemized receipt of $100,000 could be 100,000 donors, 5,000 donors, or any number in between — your guess is as good as mine,” said Kenneth A. Gross, an elections and campaign-finance expert at the law firm Skadden, Arps, Slate, Meagher & Flom.

Viveca Novak, editorial and communications director for the Center for Responsive Politics, agreed. “It's impossible for us to check claims about things like how many donors have given to a candidate and what the average amount per donor is,” she said.

This means we’re left with the federal disclosure data, which produces weaker support for Clinton’s statement.

If one defines “the majority of our support” as dollars, then Clinton is wrong — small donations accounted for 17 percent of all of her individual donations, which is well short of a majority. In fact, the Campaign Finance Institute calculated that a clear majority of the funds Clinton had raised through Jan. 31 from individuals came from donors giving $2,700 — the legal maximum that anyone can give.

On the other hand, the campaign could be right if they define “the majority of our support” as the number of donors. But this calculation isn’t transparent to outside analysis.

“Whenever a politician makes a claim about their average contribution or total number of donors, it’s impossible to independently verify,” said Michael Beckel, a reporter with the Center for Public Integrity.

Our ruling

Clinton said, “Our campaign depends on small donations for the majority of our support.”

The campaign’s best evidence for this isn’t independently verifiable. Otherwise, Clinton is wrong — small donors account for only 17 percent of the dollar amount her campaign has collected from individuals through Jan. 31.

We rate statement Mostly False.