And yet there are Republican candidates that are considering to bring a cut on social security onto the table, in order to save money and decrease the deficit.

The party is in trouble, if you asked me, they do not want to be more open-minded to bring younger people to vote for them and could cut money which effectively benefits their base. Does not make much sense, the only explanation I can find is they are being influenced by their corporate donors.
What is wrong in that? The federal debt has been ballooning to unsustainable levels in the United States. The growth has been especially steep since 2010. And a large part of this was wasteful spending, in the form of wars and additional funding for the unions. I am not sure whether the younger people would vote for a party which only provides them with freebies. If I am in their place, I would vote for a party that would improve the economy so that more better paying jobs are created. Distributing freebies will work in the short term, but in the long term it will have a negative impact.