1970s? … Try 1840s. And the 1880s. And the 1900s. And the 1930s. And …
Different minorities at different times, mind you, and not necessarily what we consider minorities right now (today a German Catholic is as American as baseball; in the 1840s the Know-Nothings considered German Catholic immigrants to be insidious invaders who would subvert American culture and democracy.)
And of course African-Americans have been part of the political process since the 1860s, even with Jim Crow. Many a Republican big-city political boss - and before the New Deal there were plenty - kept the reins of power by courting the African-American vote.
This is closer to the truth, but still leaves a bunch of stuff out. Yes, in 1920 the Dems were the party of anti-African-American racists and the Republicans were more pro-civil-rights, and yes, by 1970 those positions had flipped.
But the partys’ positions on other issues changed at different times. For example, as late as the 1980s, the Dem’s party line was more vocally anti-immigration than the Republican’s (with the exception of the occasional Pat Buchanan; but go look up Reagan or Poppy Bush on the subject, and contrast them to guys like Richard Gephardt.)
This is a misnomer, at least when viewed through the political lens and using the political vocabulary of either 2020 or 1920 instead of 1960. Neither Herbert Hoover nor Grover Norquist would view any of the mid-20th-century “conservative Democrats” or “liberal Republicans” as conservatives.
They would have described them instead as New Deal politicians, which would have been correct. FDR’s New Deal completely swept away Hoover-era small government ideology as a significant national force for 60 years: any Republicans who wanted to hang on during this flood had to sign off on the idea of a welfare state. (Goldwater’s nom in '64 was seen as a fluke, and his crushing defeat as confirmation his ideology was dead: when Nixon was elected he created the EPA and put forth a proposal for a universal basic income.)
Which is an important thing to remember. The imagined golden age of bipartisanship in the 50s and early 60s was not people remotely like today’s progressives and conservatives reaching out across the aisle. It was instead of an age of New Deal dominance, where GOPers didn’t say “I repudiate the welfare state” but rather said, “I’m really quite like FDR, but with the following changes around the edges …” Whenever a modern GOPer invokes the Good Old Days to demand Dems reach across the aisle, the answer should be, “Fine, as long as you acquiesce to 80% of my economic agenda first.”
Eventually the powerless 20th century Hoover/Goldwater small-government conservatives figured out a way to form a winning coalition: team up with the racists (who had begun migrating away from the Dems in '48 and were wooed into the GOP by Nixon in '68,) the ardent cold warriors, and the evangelicals (who had been largely absent from national political debate since the '20s, when they were pro-prohibition) to elect Reagan in 1980.