Lots of people use Medicare, Medicaid or both in any given year (say, this one). About 1 in 3 in the US. Even more people, however, are closely connected to people who use Medicare, Medicaid or both. And even _more_ people are closely connected to people who _have_ used Medicare, Medicaid or both within their memory.
As a result, big changes to these programs are difficult.
"So, when you think about the reaction to a big policy proposal like premium support for Medicare or a Medicaid block grant (or other big programs like Social Security), remember to look beyond the narrow constituencies and voting blocs and multiply, because that’s what voters will be doing."
This is a very sensible article.