May 25: This post has been updated and corrected.

When MaryAnn Nellis tried to pay for groceries on April 14, her credit card was declined. Later, she said, she found out why: Her credit card company, Capital One, had flagged an earlier purchase as potentially fraudulent. The problem? A $5 donation to Friends of Scott Walker, the Wisconsin governor's campaign committee, Nellis said.

Nellis told a Capital One representative she had not made the donation to Walker, who is fighting an effort to recall him as governor in a closely watched, expensive election set for June 5.

"Over my dead body," said Nellis, a potter and retired teacher in upstate New York who describes herself as "adamantly angry and upset" at Republicans such as Walker. Nellis disputed the charge and she was issued a new card.

Though the amount of money was small, ProPublica decided Nellis' complaint was worth following up. There have been other reports recently about insecure campaign-donation websites and the potential for fraud. Earlier this month, The Washington Times reported that Restore Our Future, the super PAC supporting Republican Mitt Romney, was using a collection system that made online donors' credit card information accessible to even amateur snoopers.

At ProPublica's request, Nellis called Capital One and asked a representative about the $5 charge to Friends of Scott Walker.

"She told me that they watch for fraudulent merchants who will put through a bunch of charges that are not legitimate," Nellis said. "I said, 'The fraudulent merchant here was Friends of Scott Walker, right?' And she said, 'Yes.' They had a little flag on any Scott Walker activity."

As an experiment, a ProPublica employee also made a $5 donation to Friends of Scott Walker on her Capital One card on May 10. Almost immediately, Walker's campaign sent an email thanking her. Less than a minute after that, Capital One emailed a fraud protection alert, saying the company "noticed potentially suspicious activity" on her account and asking her to call fraud protection as soon as possible.

When she inquired, a Capital One representative said the donation wasn't in line with her spending pattern and "our fraud department had some potential fraud concerns on the account."

Another $5 donation, made to Walker's opponent on the Capital One card, was not flagged as potentially fraudulent. Neither was a $5 donation to Friends of Scott Walker made on an American Express card. (The employee is seeking refunds of all three donations.)

We called Friends of Scott Walker for an explanation, but no one would answer our questions.

Walker's campaign spokeswoman, Ciara Matthews, emailed ProPublica on May 10 under the subject line of "follow up."

"I received a message about the story you are doing," she wrote. "The campaign does not comment on internal matters."

"How about allegations of credit-card fraud?" we wrote back. "That's hardly internal, it's external."

Matthews did not reply.

Ultimately, all we can say at this point is that Capital One appears to be flagging donations to Friends of Scott Walker as potentially fraudulent.

If anyone out there has had similar issues with online donations to Walker or other political campaigns, please let us know via email or by commenting below.

Update: Ciara Matthews, spokeswoman for Gov. Scott Walker's campaign, told the Milwaukee Journal Sentinel Monday that the charges identified as potentially fraudulent were "a result of criminals attempting to use stolen credit card information data from sources other than the campaign." Matthews also said that the campaign constantly reviews its security procedures to ensure contributors are protected. No word on who the criminals were, whether charges are being pursued or what the campaign has done to improve the security of its online donations. A phone call and email to Matthews to clarify her statement were not returned.