“If there’s anything that unifies Republicans, it’s tax reform,” Senate Majority Leader Mitch McConnell assured reporters on Tuesday who were wondering if President Trump’s latest feud with a GOP senator would threaten his top legislative priority. McConnell is undeniably correct. Tax reform, even more than repealing and replacing Obamacare, is the GOP lodestar. But the reason Republicans haven’t unveiled a tax bill, much less held a vote on one, is that they haven’t figured out how to pay for their ambitious economic plan. And on that score, the president isn’t making their jobs any easier. Like a game of whack-a-mole, Trump has taken a mallet to one GOP proposal after another for offsetting the expensive cost of reducing taxes for corporations and average American households alike. He jettisoned House Speaker Paul Ryan’s pitch for a border-adjustment tax, or BAT, that would have raised about $1 trillion over a decade. He’s wavered on the GOP’s plan to scrap the state-and-local-tax deduction after facing blowback from allies in the states that would be hit hardest by the $1.2 trillion provision. And with a single tweet on Monday, Trump popped the latest Republican trial balloon, which would have capped tax-free contributions to 401(k)-retirement accounts so that the federal government could collect more revenue up front. In each case, the president was probably playing good politics, as none of these proposals would be broadly popular. The BAT would have hit retailers who might have passed the cost onto consumers with higher prices. Millions of Americans in states like New York, New Jersey, Illinois, and California benefit from deducting their high local taxes off their federal bill. And tens of millions more take advantage of 401(k) plans, which allow employees to accrue investment earnings that won’t be taxed for decades to come. Then again, there’s probably not a single lucrative tax increase that would poll well among the nation’s vast middle class except for those that target only the wealthy or Wall Street. And that’s the essential challenge Republicans are facing as they write their tax bill: Do they want to enact comprehensive tax reform, or do they