REPUBLICANS AIM to unveil Wednesday a long-awaited tax plan, premised on the fanciful idea that slashing taxes by $1.5 trillion over 10 years will somehow leave the federal budget better off. And it is not just the GOP's most blinkered ideologues who have bought into this wishful thinking.

"I think at the end of the day this will actually be reducing the deficit because it's going to finally get this economy moving," Sen. Rob Portman (R-Ohio) said Sunday on "Meet the Press." In a sentence, Mr. Portman erased much of the credibility he developed while decrying deficits during the Obama years or running the White House Office of Management and Budget during the George W. Bush presidency.

Sen. Susan Collins (R-Maine), one of the few moderate Republicans left in Congress, was hardly more responsible. "If we have just four-tenths of 1 percent increase in our [gross domestic product], which is entirely realistic, it will cover the cost of the tax reform package," she claimed. In fact, those growth numbers cannot be assumed, and betting the federal budget on hopes of loads of new revenue is highly risky.

Just a couple days earlier, an independent report on the Republicans' most recent tax-reform framework found that the plan would wallop the federal budget, even when effects on economic growth are considered. The Tax Policy Center concluded that cutting the corporate tax rate, encouraging business investment and enhancing incentives to work would each encourage economic expansion — modestly. The extra growth would result in maybe $50 billion in new federal revenue over 10 years. To secure these small economic gains and that tiny revenue bump, Republicans would cut taxes by well over a trillion dollars, leaving a massive hole in the budget. Over time, the negative consequences of higher federal borrowing would be a serious drag on the economy.

The bill Republicans present this week will look somewhat different than the older framework the Tax Policy Center's experts assessed. But the warning should still shake Republicans who claimed to be deficit hawks when Barack Obama was president. Tax reform could be worthwhile, but only if it is paid for. Republicans such as Mr. Portman used to understand as much.

The country faces a huge funding squeeze as the Baby Boomers retire, raising pension and health-care costs. The Treasury will need ample revenue merely to maintain investments in everything else — roads, college aid, national parks, scientific research. A tax plan based on hopes, prayers and fiction puts all of that at risk.