Social Security already appears to be running aground, just two decades before the program — which accounts for about 20 percent of federal spending — is projected to crash into insolvency.

The program launched during the Great Depression — keeping millions of senior citizens from sliding into poverty — has steadily been paying out more in benefits than it collects in taxes, relying on the interest earned on federal bond holdings to help bridge the difference. Social Security costs are estimated to total $789 billion this year, paid for by $623 billion in payroll taxes.

There’s an easy repair, but it involves drastically hiking taxes, so voters aren’t hearing about it on the campaign trail. Under federal law, millionaires and billionaires get to dodge payroll taxes on a substantial percentage of their salaries. Employers and workers are charged payroll taxes on salaries up to $110,100 a year, meaning anything above that — a category that includes some of the middle class — is payroll-tax free. Simply lifting that cap would cover about 90 percent of the projected shortfall over 75 years, according to forecasts by the Social Security Administration.

"It should be talked about much more," said Eric Kingson, a Syracuse University professor and co-director of the advocacy group Social Security Works. "Most people don’t know it exists. Only 6 percent of Americans earn above the $110,100 limit. Most people are stunned when they learn that some people pay a lower rate on their Social Security contributions."

With the $2.7 trillion Social Security Trust Fund predicted to reach insolvency in 2033, at stake are the retirements of baby boomers and their children who have already watched as what retirement savings they have were hammered by the 2008 financial meltdown.

President Obama has insisted in speeches over the past year that wealthier Americans pay their "fair share" by letting income tax rates increase on those making more than $250,000. Yet, his administration temporarily cut the payroll tax during the past two years in an effort to stimulate the economy, causing a $112 billion shortfall in collected revenue for 2012.

Much of the messaging by the administration instead has been on preserving the benefits. Vice President Joe Biden said this week while touring Virginia, "I guarantee you, flat guarantee you, there will be no changes in Social Security."

Presumptive Republican nominee Mitt Romney favors increasing the retirement age and limiting benefits for high earners, but said in response to a question at the 2011 Iowa State Fair about removing the cap, "I’m not going to raise taxes. That’s my answer."

The House Ways and Means Committee introduced the cap in the 1935 legislation that created Social Security, with President Nixon indexing it to inflation in 1972 and President Carter raising it above the index in 1977 to make the tax more progressive, according to a 2011 brief released by the Social Security Administration. By 1983, the tax drew from 90 percent of earnings, compared to about 83 percent this year — roughly in line with the program’s historical average. Capturing that additional sliver of income would save the program from steep cuts, according to some Democratic lawmakers who would favor raising the cap or completely eliminating it in order to again capture 90 percent of wages.

Rep. Ted Deutch, D-Fla., introduced a bill in 2011 that would remove the cap and slightly increase benefits, but in a Republican majority House the measure co-sponsored by 44 Democrats is trapped in committee.

Sen. Tom Harkin, D-Iowa, proposed this spring as part of a broader piece of legislation to end the limit. "Social Security is the most successful program in our nation's history," he wrote in an editorial for the Huffington Post. "It is not in crisis. With sensible steps, like those in my legislation, we can make the program stronger and ensure greater financial security in retirement for generations to come."

But Senate Majority Leader Harry Reid, D-Nev., has shied away from changing Social Security. He told MSNBC in a 2011 interview that he was "willing" to take a look at reforms "two decades from now," at which point he would be pushing his 92 birthday.

The trouble with removing the cap is that Social Security would move even further away from being an earned benefit, going against the intentions of FDR when his administration formulated the program. "Roosevelt did not want the program to look like welfare," said Andrew Biggs, a former principal deputy commissioner of the Social Security Administration who is now a resident scholar at the conservative American Enterprise Institute. "It was not intended to be a highly redistributive program."

Eliminating the cap would raise an estimated $150 billion a year over the next decade, a sum that could not only strain economic growth but hit taxpayers who earn less than $250,000 a year, a group with a predominantly middle class lifestyle that Obama has vowed to shield from higher taxes.

"Your sweet spot is between $110,000 and $250,000," Biggs said. "It's not consistent with the program's history and it's also a big fat tax increase."