If he actually got the nomination this is going to completely alienate any young people that might have voted for him. A Ted Cruz nomination is going to be a win for the Democrats.
I doubt this will be the "killer issue" for the youth vote, but the Dems could make serious hay out of it for the middle age people.
The problem is, when people reach a certain age, they start to get uptight about politicians messing around with SS because they've developed their life's plans based on an expectation it will be there, even if the amount is not huge. They save up money in 401k accounts, so there is a significant net worth when it's time to retire.
When a politician promises to screw over the people who scrimped and saved the most for having a nest egg (forgoing boats and RVs along the way), then he just moved onto my "screw you" list.
I am unimpressed by people saying, "Oh, you just don't want to change your 'lifestyle'" If by that it is meant I should be OK with living in poverty because I saved my money, then that's correct. I don't want to change.