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The Federal Election Commission has asked the presidential campaign of Senator Bernie Sanders of Vermont to re-examine contributions from more than a hundred donors who appear to have given more than the legally permissible amount.

The vast majority of the donors gave several small contributions to Mr. Sanders for the Democratic primary that eventually totaled more than the $2,700 limit, according to a letter the election commission sent to Mr. Sanders on Thursday.

Such glitches are common in political campaigns, which are required to track small donors and begin itemizing their contributions when their total reaches $200. That can be harder when donors use slightly different variations of their names or contribute from more than one address. Mr. Sanders’s campaign may choose to refund the excess contributions or re-designate the excess for use in a general election campaign, when candidates can accept another $2,700.

But the F.E.C.’s review suggests that the sheer volume of small contributions Mr. Sanders is receiving — more than 3 million of them so far, according to his campaign — may be straining his campaign’s ability to keep track of which donors are which. Most of the contributions cited by the commission were given by donors with relatively unusual names, whose small checks are generally easier to tally.

In early February, after the end of the fourth fund-raising quarter, Mr. Sanders’s campaign announced that it had more than 1.3 million donors, an astonishing number for so early in the campaign cycle. And last week, the campaign announced it had received 3.25 million total donations, the most of any presidential candidate in the race. The campaign’s most recent F.E.C. filing was nearly 100,000 pages long.

In an e-mail, Michael Briggs, a Sanders spokesman, said, “We are looking into the F.E.C. staff questions and are committed to full and accurate disclosure of the tremendous grass-roots support for Sen. Sanders from people all across America.”