His Plan :
Sanders intends to expand benefits for existing retirees, while paying for the increase by taxing high wage earners. You can find the details here.
Full Disclosure :
Sanders' site has a number of fact and disclosure problems. The site claims that the plan will extend the solvency of Social Security by 50 years. The undisclosed fact is that the assessment reflects the most optimistic forecast of a fairly favorable economy. There are no guarantees.
The plan triggers 3.4 trillion in incremental spending. The vast majority of the increase does not go to seniors in need. The 3rd Way research shows that the benefits will go to those who collect the most. For every $1 of increase that goes to people in the lowest quintile of income the highest will collect $5 of increase.
Social Security's indepenedence from political priority has protected the elderly from the politics of priority for decades. This plan erodes that independence from two sides.
First, we oppose anything that connects benefits to politics. At the foundation of Social Security is the idea that benefits are earned by contribution. Sanders wants to expand benefits though politics. This draws Social Security closer to a welfare program at which point the needs of the elderly will be a political priority every year. This is a dumb idea.
Likewise we think removing the cap is a bad idea. In our opinion, exposing all wages is bad policy that will only serve to further destabilize the system over time. - (See more) This tax revenue will face continual attacks from the priority of the expense. Younger workers will want things like student loan reform. Removing the cap only erodes that independence.
In total, taxes may have to be raised, particuarly if we want to protect the system decades out. To use-up our tax resources in order to pay for a live for today mentality is simply bad policy. If Sanders wants to help the elderly fine, but let's do it with a normal welfare program.