Fighting for social security is one way to appeal to older white male voters, a key demographic that Democrats perennially lose, often by very wide margins. One thing near and dear to the heart of these older white men is retirement. And social security is the only reason most people can afford to retire. At least that is how Ross Eisenbrey phrased it here, based on retirement plans vs social security income charts. (More charts available here.)
As mentioned by Bernie sanders here, lifting the income cap on payments to social security would easily make it possible to not only save social security but even expand it. Expanding social security, perhaps coupled with new pension plan protections, would make it possible for more older workers to retire. This should create job openings, since most of the jobs in the recovery have gone to workers 55 and older. (See discussion here.)