If Trump promised to recalculate the inflation indexes that Social Security increases are based on, and to pay retoactive adjustments for the amount the indexes were understated, he’d make a major hit in the democrat’s base.
This would just be honoring the current law and commitment that was made. It wouldn’t be a new entitlement.
If Congress did that without a substantial increase in the payroll tax, you would see a 20% cut in your benefits within 10 years.
Even if nothing is changed at all, you can expect a 20% cut around 2034. That's when the Trust Fund is expected to be exhausted and by law, Social Security benefits must be lowered to what can be funded by the payroll tax.