Last month, Sen. Jeff MerkleyJeffrey (Jeff) Alan MerkleyBipartisanship alive and well, protecting critical infrastructure Overnight Defense: Senate sends 7B annual defense bill to Trump's desk | US sanctions Turkish officials over detained pastor | Korean War remains headed to Hawaii | Senators reassure allies on NATO support Dem strategist: It's 'far-left thinking' to call for Nielsen's resignation 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) SandersDems make history, and other takeaways from Tuesday's primaries Ellison wins Minnesota AG primary amid late domestic violence allegations Ironworker and star of viral video wins Dem primary for Speaker Ryan's seat 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 ObamaFormer Teacher of the Year wins Connecticut primary What happened to the Tea Party? Democrats should fully embrace their union roots 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 congressional candidate has never visited the state: AP Former Alaska senator jumps into governor race Overnight Energy: Trump directs Perry to stop coal plant closures | EPA spent ,560 on customized pens | EPA viewed postcard to Pruitt as a threat MORE (D-Alaska), as well as Reps. Linda Sanchez (D-Calif.), Ted Deutch (D-Fla.) and Gwen MooreGwen Sophia MooreIronworker and star of viral video wins Dem primary for Speaker 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 Maxine Waters is done with 'nice guy politics' 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.