Rep. Thomas Massie says it's one of the most popular bills he's ever sponsored: a proposal enticingly named the "Senior Citizens Tax Elimination Act" that would end taxes on Social Security income.

Many older Americans are shocked, the Kentucky Republican says, to learn they pay federal income tax twice: once as workers on pay diverted into the Social Security system and again after receiving retirement checks. His bill would eliminate income tax on the payout.

Originally, Social Security benefits weren't taxed. Worker contributions were. But faced with potential insolvency, Congress voted in 1983 to begin taxing the benefits of a small number of higher-income earners and recycle that money into the Social Security trust fund.

Initially, only 10 percent of people receiving Social Security income were above the threshold — $25,000 for individuals, $32,000 for couples — to pay taxes on up to half of their benefits.

But over time, inflation has led to more and more people having to pay taxes.

About 42 percent of Social Security recipients paid income taxes on some of their benefits in 2016, according to Social Security Administration spokeswoman Nicole Tiggemann, a increase of 2 percentage points from 2015.

Rather than raise the threshold to account for the declining value of money caused by inflation, Congress in 1993 added a second threshold — $34,000 for individuals, $44,000 for couples — requiring retirees to pay taxes on up to 85 percent of their benefits, rather than the initial 50 percent.

"This has been constructed kind of to be a Ponzi scheme. They are scraping money off the payments to make other payments," Massie told the Washington Examiner. He said eventually most retirees will be hit by what he considers a crafty benefit reduction.

Taxes on the benefits contributed $33 billion in 2016 into the Old-Age, Survivors, and Disability Insurance Trust Funds, according to the Social Security Administration. It's a small but significant amount, supplementing $836 billion in net contributions and $88 billion in interest.

Massie said he's heard concern that eliminating benefit taxation would make Social Security unable to pay its bills. But he points out that the bill would appropriate Treasury Department funds "equal to the reduction" to go into Social Security to compensate for the reduced tax haul. The bill also says that new appropriation cannot be offset with tax increases.

Undoing the tax on Social Security benefits isn't a new idea. It was previously championed by libertarian Rep. Ron Paul of Texas, who left office in 2013.

"It's always gotten a fair number of cosponsors but it has never really moved anywhere, and the problem with it is the typical D.C. mentality, that it would deprive the government of too much revenue," said Norman Singleton, Paul's former legislative director.

"Nobody's going to come out opposed to lowering taxes on senior citizens, they just quietly push this to the side," said Singleton, who is now president of the Campaign for Liberty.

Massie admits the bill faces an uphill battle. He said, however, that one potential opportunity is attaching it to a larger tax reform bill, which President Trump requested that lawmakers draft with simplification as a primary goal.

In the meantime, Massie said, he sees the bill as "blowing the whistle" as he works to build momentum.

"Most people don't even know this happens," he said. "When they passed it there weren't many squeaky wheels. I'm working to mobilize the growing constituency that is taxed twice on Social Security."