During Thursday's Democratic debate, Sen. Bernie Sanders (I-Vt.) refused to take former Secretary of State Hillary Clintons yes for an answer again when it came to their respective positions on Social Security, a topic that has become a concern among progressive Democratic primary voters.

Clinton pledged Friday that she would not cut Social Security benefits , claiming it was a reiteration of a position she has held all along. She was clearly responding, however, to rising pressure from progressive activists and the Sanders campaign.

The Sanders campaign did not accept her pledge as sufficiently detailed then, and candidate Sanders refused to do so again on Thursday. Sanders faulted Clinton for failing to embrace applying Social Security taxes to earnings of $250,000 or more. The Vermont senator has proposed legislation that would do just that.

Here's an area where Secretary Clinton and I believe we have a difference, Sanders said.

I would hope that you would come on board and say this is the simple and straightforward thing to do: We're asking the top one and a half percent including passive income to start paying a little bit more so that the elderly and disabled vets in this country can live with security and dignity, Sanders later added.