I think of how someone sounds after a stroke, and it can often be quite bad. Kirk Douglas comes to mind, though he has worked hard to regain his speech, but he’s never quite fully recovered it. Another is Patricia Neal, and she was rather young when she had a series of strokes, but was able to fully recover her speech. I would not be surprised if the rodents did all they could to prevent him from either debating or strictly limiting public appearances.
Thurmond was just a month short of his 94th birthday when he won his last term. He did not engage in public debates with his Dem opponent for obvious reasons, although even at 94, he was only mildly slowed. It wasn’t until his last two years in office that he became clearly feeble. The general opinion of the voting public in his last run for office was that he had earned the right to die in office. Johnson is not the institution that Thurmond was.
I don't doubt that you're correct. Assuming that Johnson does run again, and I expect he will, the Democrats will no doubt try to avoid the issues and paint him as the courageous man who cheated death and overcame tremendous obstacles just so he could continue to serve the people of South Dakota without concern for his own well being, yadda, yadda, yadda. And if the Republicans let them set that kind of tone for the campaign then they should lose. There is no reason at all why the Republicans cannot run a strong campaign based on Johnson's record and at the same time raise appropriate doubts about his ability to continue in office given his health. Given the GOP's propensity for snatching defeat from the jaws of certain victory I have my concerns.