“Here’s a difference that Secretary Clinton and I have on a very important issue,” Sanders told a room at the United Steelworkers local in Des Moines. “It is not good enough to say we are not going to cut Social Security—that is an abomination—what we must to do is to say of course we’re not going to cut Social Security, but we are going to expand Social Security.”

Sanders’ plan calls for increasing Social Security benefits to seniors by an average of $65 a month. He would pay for the plan by lifting the cap on taxable income for social security, ensuring that earners over $118,500 pay a larger share of their income.

Sanders is locked in a tight fight with Clinton in Iowa, and has increasingly been critical of her views on the campaign trail, suggesting she is in the pocket of the corporations and Wall Street banks. Clinton has countered that she has fought for progressive causes since her law school years.

Clinton often reminds voters at campaign stops that she wants to protect and “strengthen” Social Security, and that she helped lead the fight in the Senate against privatizing Social Security during the George W. Bush administration.

Sanders said that was not enough. “At a time of massive wealth and income inequality, I think it is just appropriate just right to lift that cap and ask the wealthiest people” to pay their share, Sanders said. “That is my view. If Secretary Clinton drops in, ask her her view. I don’t believe she agrees.”

This article was originally published on Time.com.