Congressman Paul Ryan, the author of the House Republican budget, is not the first politician to go long on promises and short on details. But his budget, which passed the House last week with no Democratic votes, takes untruth in taxes to a new level.

Mr. Ryan claims that he can drastically cut income tax rates without adding to the deficit, but he hasn’t specified how he would make up the lost revenue, an estimated $4.6 trillion over 10 years. Instead, he has said he would end or reduce unnamed deductions, exemptions and loopholes and defended this dodge by saying that he first wants to build consensus for the concept of pairing lower rates with fewer write-offs. That’s already a mainstay of many tax reform plans.