A little-noticed line in a recent criminal filing suggests federal prosecutors consider a popular political fundraising tactic to be legally questionable.

Why it matters: Fundraisers often boast of "5x" or other contribution matches to coax small-dollar donations. The Justice Department indicated in a court filing Monday this could amount to "material misrepresentations" if, as critics often contend, there's no evidence the match ever occurs.

• The tactic has become more prominent in recent years.
• Both parties have used it. Donald Trump's re-election campaign and its allies at the Republican National Committee exploited it to extreme degrees last year — promising up to 900% matches. The Democratic Congressional Campaign Committee has also been a frequent fan of the tactic.
• A campaign that enlisted donors to put up five times the funds raised through one such matching offer would almost certainly be inducing those donors to exceed the federal limits on their own campaign contributions.

What's happening: DOJ's comment was just a brief aside in a Statement of Offense. It accompanied a guilty plea Monday by the operator of a number of groups that raked in small-dollar donations last year with fraudulent fundraising appeals.

• One of the groups, Keep America Great Committee, sent fundraising emails that "contained material misrepresentations including promising '5x' matching of any donation to KAGC," DOJ wrote.
• It was a throwaway line in a 12-page legal filing, but one that could have major consequences for the political fundraising industry.

What they're saying: Political compliance attorney Brett Kappel called DOJ's comments about KAGC's fundraising practices"very significant."

• "This prosecution puts fundraisers on notice that the continued use of this very popular fundraising pitch will be treated as a possible violation of the mail and wire fraud statutes," Kappel wrote.

The big picture: The KAGC case caught DOJ's eye due to widespread fraud in virtually every aspect of the group's operations.

• The scrutiny of its donation-matching claims specifically, though, suggests DOJ considers such offers problematic when donations are not actually being matched.
• All indications suggest little-to-no such matching actually takes place, even among legitimate political groups.

Between the lines: Grassroots fundraising has surged for both parties, and the incentives to woo small-dollar donors are more intense than ever.

• "In many cases, email fundraisers are reaching a point of diminishing returns and are given very high goals to hit. They're doing anything and everything to reach and beat them," said Patrick O'Keefe, director of customer success at the payment processing firm Andeot.
• O'Keefe, the former executive director of the Maryland Republican Party, said tactics can include "fake matches and sketchy pre-checked recurring asks."
• He was referring to recent scrutiny of the Trump campaign and others over efforts to subtly enlist small-dollar donors into giving multiple recurring contributions.