No matter how many Republicans end up squeezing onto a campaign stage for the 2016 presidential primary debates, one thing is just about certain: Every one of them will vow to cut taxes. So, with as many as 19 contenders all repeating the same message, standing out from the crowd will be a challenge.

For some hopefuls, the flat tax has turned into a way to grab attention.

Ever since Steve Forbes put the idea at the center of his presidential campaign 20 years ago, Republican candidates — and even a Democrat or two — have latched onto the flat tax as a simple and appealing pitch to voters.

So far, at least four Republicans have talked up their support for a flat tax, or a single rate for everyone, which would replace the current system, which taxes richer people at a higher rate on ordinary income than less affluent taxpayers. To lower the rate and simplify the tax code, the flat tax also eliminates most deductions, including those for mortgage interest and charitable contributions.

“I am campaigning on a flat tax that would allow every American to fill out his or her taxes on a postcard that allows us to abolish the I.R.S.,” Senator Ted Cruz of Texas said in April. Senator Rand Paul of Kentucky and Ben Carson, a retired neurosurgeon who joined the race early this month, have also suggested a one-size-fits-all tax, while the former Texas governor Rick Perry pushed a flat-tax proposal during the 2012 presidential campaign.