Andrew G. Biggs explains at WSJ:
Bernie Sanders and Hillary Clinton have squared off in what progressive author James W. Russell recently called “the Democratic Party’s increasingly central debate over expansion of Social Security.” Challenged by Mr. Sanders to join him in promising “never to cut Social Security,” Mrs. Clinton tweeted a response: “I won’t cut Social Security. As always, I’ll defend it, & I’ll expand it. Enough false innuendos.” Nevertheless, the reforms they propose would change the program in fundamental but different ways.
Both candidates want massive tax increases, but not by increasing the payroll tax rate, the traditional means for buttressing Social Security finances. Instead, they would put all the financial burden on high-income earners. Mr. Sanders would eliminate, over 17 years, the $118,500 ceiling on which payroll taxes are levied. He also would immediately apply a 6.2% tax on investment income to households with incomes above $250,000. Mrs. Clinton is more circumspect but has recently spoken in favor of both approaches.
The maximum earnings subject to the payroll tax would be raised, but—and this is significant—without increasing the benefits eventually paid out to those who pay more in. Moreover, for the first time, non-wage income would be taxed.
What sets Mrs. Clinton and Mr. Sanders apart is how they increase benefits. Mrs. Clinton would raise retirement payments for widows as well as provide Social Security credits for individuals who take time out of the workforce to care for a child or an infirm adult. Mr. Sanders would increase the basic benefit for most retirees, raise cost-of-living adjustments, and institute a new minimum benefit for long-career low earners. Overall, Mr. Sanders’s proposed benefit increases are nearly three times as large as Mrs. Clinton’s.