I don't have to. I lived thru those years.
I even lived thru the years when republicans tried the same thing with Ronald Reagan. Calling him a democrat who switched parties.
I remember the Reagan era well as I supported him all three times for President. I don’t recall anyone having a problem with Reagan switching parties as he did so in 1964 (to support Goldwater, if now before). He didn’t switch parties (as an opportunist) to run for President.
Reagan’s opposition came from the Rockefeller Wing of the GOP (which is today known as the Bush Wing of the GOP, and attacks true conservatives).