Chris Christie is the only candidate that has a proposal for anyone to review. His plan goes beyond Social Security, and touches on Medicare and Medicaid.
The big ticket items in his proposal are means testing benefits, and increasing the retirement age of future seniors. The big cost is that he is promising to protect those in or near retirement.
Even in the best case scenario, Christie's proposal deals with roughly %50 of the shortfall. So it is a start of a plan rather than a whole plan.
While Christie continually asserts that his plan is detailed, it isn't. The big ticket item is means-testing benefits. It is unclear how he will implement his plan. Social Security has no idea what a beneficiary earns or how much wealth they have. He has already promised to protect existing retirees from any reform. It is unclear what his plan would be, much less what it would accomplish.
The other big impact in his plan comes from increasing the retirement age on future retirees. There is no fix in this solution. It simply tells future retirees how their benefits will be cut. Increasing retirement age to 69 is basically a 13% benefit cut. According to CRFB estimate (shown above), there will still be a 17% cut left over following Christie's proposal. A typical 50 year-old is better off just waiting for the cuts forced by insolvency rather than accepting Christie's cuts.