PROVIDENCE, R.I. (WPRI) — Democratic congressional candidate Sabina Matos on Monday produced evidence that her primary rival Aaron Regunberg engaged in a controversial practice aimed at skirting campaign-finance laws, arguing it proves he lied to voters.

The practice — known as a “red box” or “red-boxing” — is used to get around the federal ban on coordination between candidates and outside independent-expenditure groups supporting them, such as super PACs.

Campaigns will bury information on their public websites — sometimes, though not always, in a literal “red box” — laying out the advertising messages they need amplified and in some cases even explaining the exact voter groups that should be targeted.

Since the information is technically public, campaigns argue they aren’t illegally coordinating with the outside groups. In practice, though, The New York Times has reported that red-boxing makes “a mockery of campaign-finance laws.”

Regunberg has come under increasing attack from his Democratic rivals in recent weeks after federal filings revealed his financier father-in-law had dumped $125,000 into a super PAC called Progress Rhode Island that is sending mailers to voters on his son-in-law’s behalf. Opponents say it conflicts with Regunberg’s vocal opposition to money in politics.

Matos took the criticism a step further in last week’s debate at Roger Williams University, asking Regunberg: “Did you or do you have a red box on your website? Because if you’re against super PACs, you shouldn’t have one.”

“We don’t have a red box on our website, no,” Regunberg replied.

“Have you ever had one?” Matos asked.

“No,” he said.

“I disagree with that,” Matos responded. “I believe you had one, I can have the proof of it. … He had one.”

On Monday, the Matos campaign produced the proof: an Internet Archive capture of Regunberg’s website from July 27 showing a barely-visible link in grey text labeled “Overview” at the very bottom of the home page. (That link is now gone.)

The link led to a page headlined “Communicating to Voters” that featured typical red-box language which would have been understood by a super PAC: “Voters should see and see while on the go that while there are many candidates in this race, Aaron Regunberg is the only proven progressive with a record of building an economy that works for everybody.”

The page also said: “As early voting begins, medium-to-high propensity Democratic primary voters, particularly liberals, women, and 50+ voters across RI-01 need to read the following.” It then laid out three distinct Regunberg campaign messages, and linked to videos and photos of him that an outside group could use in its advertising materials.

Days after the “Overview” link was captured by the Internet Archive, Regunberg’s family super PAC sent out the first mailer on his behalf.

“Aaron Regunberg has consistently shown that he will say one thing and do another, just to get elected,” said Brexton Isaacs, campaign manager for Matos. “Today we learned that where there was smoke, there was fire. Aaron’s arrogance has graduated from hypocrisy to outright lies and shady coordination with his family-funded super PAC.”

The Regunberg campaign didn’t respond to a question about how Regunberg reconciled his “no” answer to Matos’s question during the debate with the language on his website July 27.

“As we said last week, when this exact same attack was leveled, our website has had numerous pages up over the course of this campaign,” Regunberg spokesperson Matt DaSilva said in a statement, insisting there was “no substantive information on the page referenced by the campaign that’s different than on any other page on our website.”

DaSilva also sought to turn the attack back on Matos, arguing that her campaign just made a blatant effort to telegraph an attack strategy to the super PACs supporting her when Isaacs released a post-debate memo laying out critiques of Regunberg and another leading Democratic candidate, Gabe Amo.

“That is a shockingly brazen level of coordination from a candidate who seems desperate to distract from the fact that her campaign, which is under criminal investigation by our Democratic attorney general, is spiraling,” DaSilva said.

The escalating war of words between Matos and Regunberg comes as outside money has been pouring into the 1st Congressional District ahead of the Sept. 5 primary. Early voting began last Wednesday.

Matos has been by far the biggest beneficiary, with roughly $800,000 spent on her behalf so far by three Washington groups — the Congressional Hispanic Caucus BOLD PAC, Emily’s List and Elect Women Vote. Most of that money has been spent on TV advertising.

In another twist on red-boxing, a media consultant for BOLD PAC went as far as to tweet out the exact details of the group’s pro-Matos spending in late July, using jargon that would be unintelligible to the average voter but potentially help her campaign make strategic decisions.

Regunberg’s outside support more than doubled on Monday, when the Working Families Party PAC disclosed spending $150,000 on digital ads to support him. Combined with the money spent by his family’s super PAC, Regunberg has received about $269,000 in third-party assistance to date.

WPRI 12 will broadcast the only scheduled prime-time TV debate in the Democratic congressional primary next Tuesday, Aug. 29, live at 7 p.m. from Rhode Island College.