Businessman Wil Cardon sent a letter on Friday to Flake requesting his signature on an "Arizona First Pledge" to bar outside groups from spending on their race to replace retiring Sen. Jon Kyl (R-Ariz.).

The pledge mirrors a groundbreaking agreement forged in January by Sen. Scott Brown (R-Mass.) and Democrat Elizabeth WarrenElizabeth Ann Warren'Fearless Girl' statue to be moved away from Wall Street bull Sanders, Warren, O’Rourke inspire patriotic small donor waves Overnight Health Care: GOP pushes stiff work requirements for food stamps | Johnny Isakson opens up about family's tragic loss to opioids | Republicans refuse to back vulnerable Dem's opioids bill | Dems offer new public option plan MORE that has defied the low expectations of many skeptics. The Brown-Warren agreement stipulates that if any outside group airs an ad benefiting one candidate, that candidate must donate 50 percent of the cost of the ad to a charity of his or her opponent's choosing.

When Flake's team declined to sign on, Cardon sent his supporters a fundraising pitch calling out Flake and challenging him to reverse course.

"As Congressman Flake and I both care deeply about Arizona and the future of this country, I believe we should establish the same pact for our Senate race," Cardon wrote in the email. "However, it appears as if I stand alone in this sentiment."

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Banning spending by outside groups would have very different effect on Cardon than on Flake. Cardon raised $3.2 million in the first three months of 2012 — an extremely impressive figure for a non-incumbent candidate, except that $3 million of it came from a loan from Cardon himself.

"First, Wil Cardon needs to stop illegally funding his campaign before making pledges, but second, we will urge no outside spending as soon as he stops trying to buy this race with his inheritance," said Flake spokesman Andrew Wilder.

Flake's campaign did not respond to an inquiry seeking clarification about the accusation. But the remark appeared to refer to a Federal Election Commission complaint filed against Cardon by one of Flake's supporters. Cardon's campaign has dismissed that complaint as frivolous and politically motivated.

Cardon spokeswoman Katie Martin noted that Flake too has been hit with an FEC complaint, and said Flake's tactics revealed that he has become a career politician.

"Congressman Jeff FlakeJeffrey (Jeff) Lane FlakeHeitkamp becomes first Dem to back Pompeo for secretary of State Duckworth brings her baby to Senate vote, drawing a crowd Senate confirms Trump’s pick to lead NASA MORE's continued refusal to sign this pledge again shows how out of touch he is with Arizona," said Martin. "Whether it's his support of amnesty, cap and trade legislation, taxpayer perks or his D.C. insider status, it's crystal clear that he doesn't understand the problems Arizonans face and is willing to sell their vote to the highest bidder."

In total, Cardon has injected more than $4 million of his own money into the race, making him one of the top self-funders of the cycle.

Flake, meanwhile, raised almost $950,000 during the first quarter. The Democratic recruit, former U.S. Surgeon General Richard Carmona, brought in about $800,000 in his first full quarter in the race.

- This post was updated at 9:48 p.m.