His plan will . (AP Photo/Jim Cole)

Sen. Rand Paul (R-Ky.) will call for the tax system to be overhauled so all Americans, regardless of income, are taxed at a single rate, according to an adviser. And he's not the only rider on the flat-tax train.

"It's an idea that [candidates] think has a little bit of sexiness to it," said James Pethokoukis, a scholar at the conservative American Enterprise Institute.

Paul said in February that he plans to unveil the "largest tax cut in American history" and has vowed to get the "IRS out of your life." Paul's plan is "90 percent done," said Stephen Moore, who has been helping Paul with the plan -- and it will include a flat tax.

"He is going to have a very low rate flat tax that is going to be the lowest rate flat tax that I think anyone is going to propose," Moore said. "We're looking at a rate as low as 14, 15, 16 percent."

Moore said it will be a "huge job creator" and will amount to a tax cut for working and middle-class people.

A spokesman for Paul would not comment on the plan, saying it is not yet ready to be released.

Sen. Ted Cruz (R-Tex.) wants to go one step beyond and abolish the IRS. He jokes on the campaign trail that someone should put the tens of thousands of people who work for the agency on the southern border.

"Instead of a tax code that crushes innovation, that imposes burdens on families struggling to make ends met, imagine a simple flat tax," Cruz said in his presidential announcement speech in March, "that lets every American fill out his or her taxes on a postcard."

Cruz's campaign said that he will release a tax plan -- eventually.

They're not the only Republican presidential contenders feeling good about a flat-tax pitch. Jeb Bush said he is open to the idea, though his eventual blueprint will lead to "bigger and broader tax relief." Ben Carson endorsed the idea of a flat tax in Iowa last week. Rick Perry continues to push for a flat tax. Wisconsin Gov. Scott Walker (R) has pitched it. So has Ohio Gov. John Kasich.

Of course, it's far from a new idea. Republican Steve Forbes made it a centerpiece of his 1996 campaign, using his considerable wealth to bankroll his campaign for the presidency and a flat tax in both 1996 and 2000.

The idea remained on the fringes of the Republican Party until the 2012 election, when three Republican candidates -- Perry, Herman Cain and Newt Gingrich -- all called for a flat tax. Cain's had the catchiest slogan, the 9-9-9 plan: a 9 percent sales tax, 9 percent income tax and 9 percent corporate tax rate.

Candidates embracing a flat tax this year claim it will help middle class families and spur growth. Pethokoukis doesn't agree.

"Whenever you look at the math, it doesn’t work," he said, arguing that it would force a president to collect income taxes from the 43 percent of Americans who don't pay them each year, ultimately raise taxes on the lower and middle classes, and result in cuts to entitlement programs. But, he said, it sounds good on the campaign trail.

"These candidates think that is an idea that will supercharge their candidacy even if it won’t supercharge the economy," he said. "It will play to some of the worst stereotypes of Republicans, that they only care about cutting taxes and they don’t care about the deficit and they don’t care about the middle class."