Republicans in Congress are plowing ahead on tax reform, and one obstacle is the complexity of Senate budget rules that limit how much taxes can be cut. The good news is that for once Washington’s fiscal fictions could be deployed to improve policy by repealing ObamaCare’s individual mandate as part of tax reform.

The Senate Finance Committee on Thursday released the details of its tax proposal, which includes a permanent 20% corporate rate and more. Senators Pat Toomey and Bob Corker cut a budget deal to allow for $1.5 trillion...