Vermont Sen. Bernie Sanders' presidential campaign slammed South Bend, Ind., Mayor Pete Buttigieg for what it called a "gimmick" for help lower its average campaign contribution amount by the end of the year.

It came amid new attack lines from various Democrats seeking the White House, claiming some of their rivals were seeking too many campaign contributions from the wealthy. Sen. Elizabeth Warren, D-Mass., called out Buttigieg during last week's debate for holding a recent fundraiser at a California "wine cave," adding, "Billionaires in wine caves should not pick the next president of the United States." Buttigieg fired back by saying he was the only candidate on the stage who was neither a millionaire nor a billionaire, telling Warren, "this is the problem with issuing purity tests you cannot yourself pass."

On Tuesday evening, the Pete for America Innovation Team sent out an email to supporters launching a "contest" in which the person donating the smallest unique amount would win a prize.

"All you have to do to win is donate the smallest amount that nobody else donates," the email read. "In other words, suppose you donate $1.00. If someone else playing also donated exactly $1, you both lose. We'll see if only one player donated $1.01 and so on until we find an amount donated exactly once, and that's our winner."

The email added that "multiple donations are allowed" and that "hopefully, this is a fun way to contribute" to the campaign.

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Tim Tagaris, a campaign adviser for Sanders I-Vt., mocked Buttigieg's efforts.

"The Pete for America Innovation Team out there working hard on Christmas Eve coming up with gimmicks to lower his average donation amount this quarter. Funny stuff," Tagaris tweeted. "This is so transparently hilarious. Wow, his average donation was lower this quarter... it’s a Christmas miracle!"

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The Buttigieg campaign did not immediately respond.