Mitt Romney, who built Bain Capital LLC and ran Massachusetts with technocratic detachment, is a famously astute numbers guy. These twin achievements make it all the more frustrating that Romney has resisted filling in the details on the signature proposal of his U.S. presidential campaign: to cut taxes by 20 percent across the board without raising taxes on the middle class or exploding the deficit.

Romney’s reticence has led others to do the job for him. This in turn has made his campaign unhappy. An analysis from the nonpartisan Tax Policy Center has met with particular scorn as it concluded Romney’s plan is mathematically impossible -- that there simply aren’t enough deductions, credits or loopholes in the tax code to offset the cuts for the well-off without wreaking havoc. (Romney’s promise to increase defense spending only complicates matters.) As Bloomberg View’s Josh Barro has demonstrated, efforts to poke holes in the tax center’s work are unconvincing.

The desire for specificity is hardly unreasonable. Running in 2000, George W. Bush insisted that his proposed tax cut would be a boon to the middle class. Experts demurred, arguing that the top 1 percent of income earners would reap a windfall. Like Romney, Bush declined to show his math. In the end, his 2001 tax cut delivered almost half of its benefits to the top 1 percent and initiated Bush’s march toward a trillion-dollar deficit.

At his debate with Vice President Joseph Biden last week, Republican vice presidential nominee Paul Ryan said the campaign wouldn’t release definitive numbers because they would only be finalized in bipartisan negotiations with Congress. Tax and spending legislation must work its way through Congress, of course. But it’s hard to give Romney and Ryan the benefit of the doubt when they trumpet big tax cuts while steadfastly refusing to disclose who will pay for them.

In his first debate with President Barack Obama, Romney made a decisive and welcome pivot to the ideological center. We would like to imagine this moderate Romney spelling out the details of a sensible tax reform plan that limits deductions, adjusts rates and calls for sacrifice from all Americans. Such a stance would surely put Romney in a stronger position to ask Obama about the president’s own plans. Details matter. If Romney has some secret sauce that tax experts have overlooked, now is the time to tell us. Otherwise, we are left to fill in the blanks.

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Today’s highlights: the editors on how Spain can stop Catalonian secession; Edward Glaeser on the winner of the economics Nobel; Jeffrey Goldberg on Republicans using the Benghazi attack to undermine Obama; Michael Kinsley on checking candidates’ arguments instead of facts; William Pesek on “Gangnam Style” and South Korea’s economy; Ramesh Ponnuru on why both candidates are wrong on China; Carmen M. Reinhart and Kenneth S. Rogoff on why U.S. financial crises aren’t different.

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