I agree.
So, given that that it is clearly tantamount to political suicide for the GOP, are the supportive Republican legislators naive or “in” on the scam? And, if they’re in on it, why?
Some are and some aren't. Many Reps see the writing on the wall in terms of demographics. Half of all children 0-5 are minorities, i.e., asian, black, and Hispanic. The Dems will benefit from the changing face of America, at least in the short term. So some Reps figure that leverage is going to decrease and better to try to strike the best bargain they can while they still have a country where "minorities" don't form a majority. It is more complicated than that, but close enough for government work.
There are also some naive Reps who actually believe they can trust the Dems and negotiate deals with them. It is like Charlie Brown and Lucy with the football. We always act surprised when they pull the football away at the last minute.
They think they can get elected by appealing to business interests who want it plus immigrant groups. A Republican doesn't have to run as a conservative. (Think of Nelson Rockefeller.) Note there are many businessmen who want national health insurance so as to relieve them of the expense of insuring their employees. I can conceive of quite a few Republicans who would support national health insurance because they thought it would enhance US competitiveness.