A decade ago, the push to privatize Social Security looked dead in the water. Fresh off his re-election, then-President George W. Bush put the issue at the top of his second-term agenda. Even so, he couldn't get enough votes in Congress to turn his vision into a reality.

However, the 2016 Presidential campaign is proving that not everyone has given up on the notion. Several GOP candidates, including Governor John Kasich of Ohio and Senator Rand Paul of Kentucky, want to allow workers to put at least some of their payroll tax dollars into accounts that they own and control.

Perhaps it's not surprising that some on the right are still hoping to fundamentally reshape Social Security. One of the primary arguments for privatization is that the only way to keep the program solvent is to increase the rate of return on payroll deductions. And Social Security's fiscal health is in worse shape today than it was back in 2005.

The current system is essentially an intergenerational transfer of income. The money that active workers put into the program through the payroll tax helps pay the benefits of today's retirees. (See Introduction to Social Security to learn more.)

To date, the pay-as-you-go model has worked pretty well because there have been more than enough Americans in the workforce to pay retirees. For a number of years the program took in more than it was paying out, generating a sizable reserve that still exists today. The program invests these extra dollars in Treasury bonds, generating additional interest income for its coffers.

But demographic shifts are throwing a wrench in the system. The ratio of workers to retirees is dropping, and by 2020 the Social Security Administration projects it will start running annual deficits that will cut into its reserves. That yearly shortfall is expected to last for the duration of the program's 75-year forecasting period.

The SSA estimates that its surplus will run out by the year 2034, at which point it will only bring in enough revenue to pay about 75% of scheduled benefits.

Figure 1. The Social Security Administration estimates that program expenditures will start to exceed income in the year 2020.

: Social Security Administration

The solution, some say, is to let workers divert part of their paycheck toward private investment accounts that they control. They could use the funds to purchase some combination of stock, bond or money market funds, depending on their risk tolerance and objectives.

Once workers hit retirement age, they could choose to purchase an annuity that would provide a lifetime stream of income. Any funds not withdrawn or used for an annuity could be handed down to their heirs.

The advantage, advocates say, is that individually directed investments have the ability to vastly outperform the centrally managed trust fund, which by law can only invest in Treasury bonds. Historically, stocks have generated an annual return of roughly 10%, on average. Treasuries, which are backed by the full faith and credit of the U.S. government, generally return less than bonds, which have an average yield of about 5.5% per year.

In theory, then, money that's socked away in a private account has the potential to go farther than money put into the Social Security trust fund. What's more, retirees wouldn't have to worry about demographic trends, since their retirement kitty would no longer be tethered to the size of the current workforce.

Of course, there's another reason that many Republicans embrace privatization. It fits into their overarching narrative that more power should rest with the individual and less with government.

The basic goal of Social Security is to provide a financial safety net for Americans as they reach advanced age (as well as for those with disabilities that hinder their ability to work). Critics of privatization argue that subjecting those funds to the ups and downs of the market goes against that very objective.

After the country's experience of the Great Recession, Republicans may have an even harder time discrediting this argument than they did in 2005. Can workers mitigate their risk by shifting to more conservative investments as they reach retirement? Sure. But not every American has the financial literacy to do so.

Perhaps an even bigger hurdle is figuring out how to pay for the transition away from the existing pay-as-you-go paradigm. Even as younger workers are given the right to put tax dollars in a personal account, the government still has the obligation to pay current retiree benefits. That gets expensive.

Democrats say the only way to make it work would be to scale back payments to existing Social Security recipients or to raise the payroll tax. The former has been a third rail for those on the left, and the latter is unlikely to gain support from the right.

A gradual transition, which some Republicans have called for, softens the blow somewhat. Under this strategy, workers would start by putting only a small portion of their salary into a private account and then increase their allotment over a period of years. But even that approach isn't without significant costs.

Private accounts aren't the only way to handle the impending shortfall. Other ways to shore up Social Security include raising the cap on taxable earnings to generate more revenue. For 2015, any income exceeding $118,500 is not subject to payroll taxes. On the expense side of the ledger, one commonly floated idea is to rein in liabilities by increasing the retirement age.

Another approach would be to allow the trust fund to become more aggressive in its investments. Instead of solely buying Treasury instruments, some say it should be allowed to use a portion of its assets on stock purchases.

Advocates say it's a best-of-both-worlds approach. First, there's the potential for higher returns. And second, unlike private accounts, you're leaving investment decisions to an institution that knows how to manage risk better than most individuals do.

It's still very much an uphill climb for those who want to privatize all, or even part, of Social Security. However, the idea of controlling your own retirement money is one that continues to hold appeal for a segment of voters.

For more on the topic, see Social Security Depletion: Is the Fear Justified?