The Senate may vote as soon as this week on tax reform, and the outcome hangs on a few GOP holdouts. Two worries of the fence-sitters are how much the reform will improve the economy and whether it will add to the federal deficit. Let’s examine these concerns against the budget math and economic evidence.

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Start with the fact that the GOP budget outline allows for a net tax cut of $1.5 trillion over a decade on a statically scored basis thanks to a deal brokered by Senators Pat Toomey and Bob Corker. Democrats and their...