IRS Commissioner John Koskinen is leaving a good-bye gift as he exits his job tomorrow and it is going to be a "major hit on employers," according to an editorial in this morning's Wall Street Journal.

The IRS will for the first time begin to enforce the unpopular employer mandate in ObamaCare. Letters to noncompliant employers with more than 50 employers are in the mail. The mandate was supposed to be enforced in 2014 but Koskinen said that the agency did not possess the resources.

Now, with political fallout from enforcing the mandate no longer a threat to the Obama administration, Koskinen has found the resources. The effects could be devastating to businesses that have the requisite number of employees but lack the resources to comply.

The Wall Street Journal explains:

Nobody knows how many businesses will be hit, but by some estimates it will be tens of thousands. The Congressional Budget Office estimated in 2015 that employers would owe $9 billion in fiscal 2016 and $13 billion in fiscal 2017. This comes as premiums continue to rise on the ObamaCare exchanges.

The mandate is particularly burdensome for smaller job creators hard-pressed to comply with an extraordinarily complex mandate that requires them to report on the status of every employee and dependent. The letters are also a blindside hit, since the IRS was silent about its plans.

Treasury Secretary Steve Mnuchin could have overruled Koskinen's decision to enforce the employer mandate but declined to do say. A statement from the Treasury said that Treasury lawyers had found no grounds to do so, though the editorial suggested that it would not have been stretching the law for Mnuchin to have done so, thus giving businesses more time to prepare.