Last month, Sen. Jeff MerkleyJeffrey (Jeff) Alan MerkleySanders, Harris set to criss-cross Iowa Dem senator calls for US action after 'preposterous' Saudi explanation Graham: Saudi’s findings on slain journalist not 'credible' MORE (D-Ore.) announced plans to introduce a bill to increase the annual cost-of-living adjustments for Social Security.  He proposed to pay for it by raising the Social Security payroll tax rate of the wealthiest Americans -- those who make more than $250,000 per year -- closer to the rate already paid by middle and working class American workers.

Many people don’t know that any income above $117,000 per year is not taxed by Social Security (this limit on the amount of earnings subject to the tax is adjusted annually to keep up with inflation). That means that someone who makes twice the cap this year – $234,000 – pays the tax on only half of his or her wages. And those lucky enough to make at least $1.2 million per year are taxed by Social Security on less than one-tenth of their income.

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In other words, workers who make $117,000 or less per year – the vast majority, as a recent analysis by the Center for Economic and Policy Research shows -- pay a higher Social Security payroll tax rate than the 5.6 percent who make more.

Merkley will be joining Sen. Bernie SandersBernard (Bernie) SandersElection Countdown: Small-donor donations explode | Russian woman charged with midterm interference | Takeaways from North Dakota Senate debate | O'Rourke gives 'definitive no' to 2020 run | Dems hope Latino voters turn Arizona blue Bernie Sanders' age should not disqualify him in 2020 Small-dollar donations explode in the Trump era MORE (I-Vt.) and Rep. Peter DeFazio (D-Ore.), who also have introduced bills to apply the Social Security payroll tax to wages above $250,000. These bills are similar to a proposal by Barack ObamaBarack Hussein ObamaRepublicans bail on Coffman to invest in Miami seat Five takeaways from the first North Dakota Senate debate Live coverage: Heitkamp faces Cramer in high-stakes North Dakota debate MORE during the 2008 presidential campaign. The Social Security Administration’s Chief Actuary estimates that the payroll tax cap sections of these proposals would reduce the program’s long-term budget shortfall by about 80 percent.

Others have proposed increasing or phasing out the Social Security payroll tax cap entirely, a concept popularly coined as “Scrap the Cap.” Sens. Tom HarkinThomas (Tom) Richard HarkinOn Nicaragua, the silence of the left is deafening Dem Senator open to bid from the left in 2020 Senate GOP rejects Trump’s call to go big on gun legislation MORE (D-Iowa) and Mark BegichMark Peter BegichAlaska governor Walker suspends reelection campaign Alaska's lieutenant governor resigns over 'inappropriate comments' Republicans see silver linings in deep-blue states MORE (D-Alaska), as well as Reps. Linda Sanchez (D-Calif.), Ted Deutch (D-Fla.) and Gwen MooreGwen Sophia MooreHouse lawmakers introduce bill to end US support in Yemen civil war Ironworker and star of viral video wins Dem primary for Paul Ryan's seat On The Money: Trump defends tariff moves as allies strike back | China says it's ready for trade war | Maxine Waters is done with 'nice guy' politics | ZTE allowed to resume some operations MORE (D-Wis.), have introduced bills that would phase out the cap over five to ten years. The phase-outs of the cap in these proposals are estimated to eliminate 70 to 80 percent of the long-range shortfall.

While every one of these senators and representatives earn over $117,000 annually, Census Bureau data shows that only about 1 in 18 workers would pay more if the cap were scrapped, and only the top 1.4 percent (1 in 71 workers) would be affected if the tax were applied to earnings over $250,000.

It gets even more interesting when you look at different states and slices of the population.  In the home states of Merkley, Harkin and Sanders (Oregon, Iowa, and Vermont), the top 4.2 percent, 3.5 percent and 4.0 percent of workers, respectively, would pay more if the Social Security payroll cap were phased out.

Even fewer women workers would be affected if the cap were abolished: only about 1 in 36 (2.8 percent) of them would pay more, and the top half of one percent would be affected if the tax were applied to earnings over $250,000.  Similarly, only about 1 in 50 black or Latino workers would pay more if the cap were lifted entirely, and about 1 in 200 would be affected if earnings above $250,000 were subject to the tax.

As the retirement security of working Americans continues to be an important topic of debate, these proposals -- to strengthen Social Security by having the wealthiest Americans pay the same payroll tax rate as the rest of us – deserve the utmost consideration.

Woo is the director of Domestic Policy at the Center for Economic and Policy Research, a progressive economic think-tank.