Sen. Susan Collins (R-ME) has crunched the numbers on repealing Obamacare’s individual mandate, and she does not like what she sees.

The morning after Senate Republican leaders gave their blessing for the tax bill to include a provision gutting the mandate, the Maine senator told reporters that her staff used data from the IRS, the Congressional Budget Office, and the Kaiser Family Foundation to calculate that the large increase in health insurance premiums that would result from the mandate’s repeal would more than cancel out the tax breaks many middle class Americans would get from the rest of the GOP tax bill.

“I am no fan of the individual mandate and I very much want to see tax reform,” she said slowly and carefully. “But I believe taking a particular provision from the Affordable Care Act and appending it to the tax bill greatly complicates our efforts. One of my concerns is that it will cause premiums in the individual markets to go up as healthier, younger people drop out.”

“I have new statistics,” Collins added, patting a thick black binder under her arm, “that show that for some middle-income people, it will cancel out their tax cut. The increased premium would be more than the tax reduction they would get from this bill.”

A new report from the non-partisan CBO found that repealing the individual mandate would increase premiums at least 10 percent every year for the next 10 years.

Collins says she plans to present this data to her Republican colleagues in a closed-door lunch on Wednesday. She declined to give a definitive answer whether the inclusion of the mandate’s repeal will cost leadership her vote on the tax bill, and said she hopes to still be able to shape the final product before it comes to the Senate floor.

“The bill keeps changing, even as we speak,” she said with a smile.

Collins, who arguably saved the Affordable Care Act multiple times this year, and whose state recently voted overwhelmingly to expand Medicaid, said it will be “very difficult” for her to support the bill even with many other provisions she likes. The new report from her staff, which she proudly noted includes two tax lawyers and an economist, has further convinced her that repealing the mandate would be a recipe for disaster.

“I suspected this based on what I know about insurance markets, but now I have the actual data,” she said.

If Collins and any two other GOP senators vote against the tax bill, it will see the swift demise of every other Obamacare repeal bill put forward this year.