Marco Rubio and Mike Lee’s long awaited tax plan, which the senators released two weeks ago, hits many traditional conservative goals like eliminating capital gains taxes, consolidating tax brackets and reducing the corporate rate, along with a major expansion of the Child Tax Credit (CTC).

The plan received positive reviews from many on the right, and a moderately positive one from me. Though the plan would balloon the deficit and the CTC expansion would leave out low-income and non-traditional families, I wrote that Rubio and Lee "have put forward a credible, conservative tax proposal that could lay the groundwork for comprehensive tax reform in the future."

On further thought, the plan's main weaknesses are far worse than I originally acknowledged.

Rubio-Lee is indeed a credible, conservative tax plan. For the last six years, Republicans have been offering unrealistic tax plans. Herman Cain's 9-9-9 plan would have imposed a 9 percent tax on transactions, a 9 percent income tax, and a 9 percent sales tax. Mitt Romney's tax plan in the 2012 presidential election would have cut all tax rates by 20 percent and cut the capital gains rate for all but the richest Americans while making up the revenue by closing unspecified tax breaks. Both of those plans would have been incredibly regressive. But Cain and Romney both insisted their plans would be deficit neutral, which appeared mathematically impossible. Romney and Cain's tax plans were completely detached from the realities of our tax system. They were all about politics, an attempt to convince GOP primary voters that they could fulfill impossible conservative goals.

In many ways, Rubio-Lee is a more conservative plan than those two. For instance, it eliminates the capital gains tax altogether. The same goes for the estate tax. But Rubio and Lee at least made their plan mathematically realistic: They dropped the pretense that they could be deficit neutral, and admitted that the plan will cut revenues by trillions of dollars in the first 10 years. That makes Rubio-Lee less a political plan than a legitimate policy document.