The conservative group Americans for Prosperity is defending a television ad claiming Obamacare jeopardized the life of Julie Boonstra, a Michigan cancer patient, after fact checkers raised concerns over its accuracy. But the Koch brothers-backed AFP is still refusing to provide documented evidence to back up its characterization of her story.

The ad targets Michigan Rep. Gary Peters, a Democrat running to succeed retiring Democratic Sen. Carl Levin. It features a leukemia patient, Julie Boonstra, who says that because of Obamacare her out-of-pocket health care costs have become unaffordable after her previous plan was canceled – meaning she might not be able to obtain cancer medication necessary to keep her alive.

What the ad didn’t mention is that Boonstra had obtained new coverage through her state’s Obamacare health care exchange in a network that included her physician. In addition, the monthly premiums under her new plan were cut almost in half, from $1,100 a month to around $568.

The drop in price for Boonstra’s monthly premium is especially significant because the savings over a year are almost identical to the maximum $6,350 that insurance companies can legally charge an individual for out-of-pocket expenses under Obamacare. That means, on an annual basis, it’s virtually impossible for Boonstra to pay more for care under her new plan.

Lawyers for the Peters campaign contacted managers at the TV stations running the ad and asked them to obtain substantiation for the ad or pull it down. In response, AFP sent a document to the stations that provided no new information about Boonstra’s situation, but instead directed them to a Politico story from September on how some consumers faced higher deductibles and co-pays under their new plans.

When the Washington Post’s Glenn Kessler asked AFP spokesman Levi Russell last week about the apparent discrepancy between the ad’s claims and Boonstra’s actual insurance plan, Russell didn’t deny that Obamacare’s caps would prevent Boonstra’s annual costs from going up. Instead, he said the primary problem was that Boonstra’s costs in any given month before she hits the $6,350 limit could be too high to pay all at once.

This new explanation may have thrown Fox News’ Steve Doocy on Monday, who asked Boonstra in atelevised interview to explain her “new policy with premiums of $1,100.” Boonstra corrected him, saying her new plan actually cost much less but that she was afraid of a new payment schedule.

“Under my old policy, I knew what I could afford every single month because I wasn’t hit with extra charges,” she told Doocy. “Now, I don’t know what I have to pay month to month based on that out-of-pocket expense. Leukemia tests are extremely expensive and I just don’t have the five or six thousand dollars in the bank to cover that expense.”

Boonstra may well be frustrated with having her old plan cancelled, just like many people whose plans were cancelled under Obamacare. And she’s clearly worried about paying higher expenses under the new one. But one of the ad’s central claims is that Boonstra, right now, cannot afford lifesaving treatment thanks to higher costs.

“Now, the out-of-pocket costs are so high, it’s unaffordable,” she said in the ad. “If I do not receive my medication, I will die.”

As Kessler noted in his fact check, it’s impossible to evaluate AFP’s claim that Boonstra’s month-to-month costs have gone up, let alone to the point that they actually threaten her health, without knowing what Boonstra’s bills are and what her payment schedule is. Russell suggested to msnbc that the group was unlikely to furnish such documentation.

“Given that your past reporting on this appears to be no more or less than an attempt to smear and discredit Julie, you can imagine I’m not rushing to provide you any information,” Russell told msnbc in an e-mail.

He added, “After being lied to and having her plan cancelled, Julie has made a reasonable judgment that her new plan is unaffordable due to ongoing unexpected out-of-pocket costs that have been impossible to budget for. That simple concept is one that most people readily understand.”

Boonstra did offer some clarification in an interview with The Dexter Leader, saying she was upset to find a prescription that was once covered is no longer in her new plan. But she added that she wasn’t sure yet how her overall costs had changed.

“I truly would love to show the public my numbers, but like I said I just don’t have that because I haven’t had those visits,” Boonstra said. “People don’t have that certainty – they don’t have the stability of knowing every month what they’re going to be paying now and it’s the ability to actually have that sum of money to pay.”

No doubt, it’s an extremely stressful experience: In addition to navigating a new plan while facing a life threatening illness, Boonstra said she briefly went without coverage due to technical problems with the health care website.

But the ad’s central claim isn’t about a glitchy website or general anxiety about bills, it suggests her care has become “unaffordable.” By Boonstra’s own account, it sounds as if she’s still figuring out where and how much her expenses have changed. And the new debate over month-to-month versus annual expenses – which AFP raised only after the ad had already aired – is a big step away from the initial TV spot, which implied that under Obamacare she might have lost her old doctor or that she faces higher expenses because she has no health plan at all.