To wit, did he propose it, knowing that the Republicans are much like Pavlov's dogs, only not so bright, and the moment he proposed something they could be counted on to reject it?
That would be a very clever way to dispose of this proposal.
He could count on progressives to oppose it. He could count on conservatives to oppose everything. He may have disgruntled progressives, but he is not running for re-election, and waking up his supporters means they are awake later.
My remarks are a bit short, but it is hard to produce evidence, and what I am saying is a bit different than many other remarks here.
Having said that I add a modest anecdote. As some of you know, I am a Libertarian. In 2007-2008, I moderated two Presidential candidate debates for the folks running for my party's Presidential nomination. Our leading candidate's position was that Social Security is a contract, and when you have a contract, you try to keep it.
I urge everyone to read the GAO report on Stewardship. Short form: Retirement and Disability are in good shape, perhaps with modest tax adjustments. Medical Care, unless cost increases change drastically, is going to cause serious issues.