I see that ridiculous argument online, too. It's the "big lie." There's a kernel of truth in it, but only enough that it needs some explanation.
Of course, liars rely on that. It's far easier just to repeat the lie over and over and over again than to explain why it's a lie,... and to keep on explaining it. It takes a lot less time, too.
At the same time, even if it were true (which, again, it's not), it wouldn't excuse anything. If Republicans are racist, it's not an excuse to say that the Democrats are also racist - even if it were true.
That's how a three-year-old argues. "Well, Bobby does it, too!" That's not an excuse. If I kill someone, will I escape blame if I point out other people who are also murderers? I doubt it.
For more than a century, the South was solidly Democratic. And, yes, the Ku Klux Klan was Democratic, too. They were still racist and still conservative. The KKK was never 'progressive' (admittedly, political issues - and the labels we use - change as time goes on, too).
But the rest of the Democratic Party changed. Finally, they started doing the right thing. This probably started with Franklin D. Roosevelt - or Eleanor Roosevelt, who seems to have persuaded him - when black people were permitted to take part in New Deal programs, instead of just white people.
Then Harry Truman desegregated the military. Even the initial steps towards that almost cost him re-election in 1948, as Strom Thurmond - yes, the future Republican - ran for president on an explicitly segregationist platform and almost won enough votes in the South (which was ordinarily a gimme for the Democrats) to cost Truman the election.
Incidentally, that's where this photo comes from. Truman was popular enough that he would have won re-election easily without the racist revolt of southern Democrats. As it was, he barely won. It was a clear warning from the Dixiecrats.
Even then, those Dixiecrats couldn't bring themselves to vote Republican (the 'party of Lincoln,' you know). And our next president, Dwight D. Eisenhower, wasn't any better, from the standpoint of those southern racists. So it wasn't as though they really had an alternative,... yet.
Then came Lyndon B. Johnson and the 1964 Civil Rights Act. Yes, as Republicans will tell you, many Republicans voted for it,... and those racist Dixiecrats did not. But it was a southern Democrat (Johnson was from Texas) who pushed it through Congress.
President Johnson, and the northern Democrats who supported it, knew that they'd lose the South. They did the right thing, anyway. Say what you will about the Democrats - and their timidity, their fecklessness, their lack of political courage frustrates me to no end, sometimes - this time they did the right thing, knowing that it would hurt them politically.
Republicans reacted with glee. The Republican Party eagerly did the wrong thing, because it would benefit them politically. GOP leaders began their notorious 'Southern strategy' of deliberately wooing white racists.
It worked great. Southern white racists had an option now. And very, very quickly, the South went from being solidly Democratic to solidly Republican. They were still the same racists, of course. They hadn't changed their minds, just their political party. Strom Thurmond was still the same guy he'd always been.
Well, not entirely, of course. As society changes, even conservatives are dragged along, kicking and screaming all the way. And as a matter of politics, there wasn't much chance of getting segregation restored, anyway (although they were able to maintain effective racial segregation in schools and neighborhoods, even if it had to be disguised a bit).
Besides, those Republican politicians weren't wooing racists for the racists' advantage. They wooed racists for their own political advantage. For the most part, they just threw the racists a bone occasionally, while they used their new political power for what they really wanted to do (primarily, to cut taxes on the rich). It was mostly just rhetoric. It's always easy to say what you want to do, while not planning to actually do it.
In the Reagan years, Lee Atwater explained how it was done. Republicans discovered that racism worked very well in the North, too, as long as they weren't too blatant about it. In the North, they convinced working-class whites to see economics in racial terms, thus getting them to vote against their own best interests. Those were the so-called "Reagan Democrats," and many of them remain in the Republican Party to this day.
As time went on, though, this changed the Republican Party. They'd quickly lost the black vote, of course. And they lost most of the Northeast (except Wall Street, for obvious reasons), which had formerly been the Republican stronghold. Politically, the South more than made up for that, but racism wasn't the only thing the South brought into the Republican Party.
Believe it or not, the Republican Party never used to be anti-science. Indeed, there used to be about as many scientists registered Republican as there were registered Democratic (about 40% in both cases, with the rest Independents). After all, there's no real reason why a scientist wouldn't be conservative economically, socially, or even politically.
But the racists who flooded into the Republican Party were often fundamentalist Christians, too - people who had little respect for science and were frequently antagonistic to it. As time went on, scientists began to leave the GOP, just as those people who had a problem with using racism for political advantage did. Over time, the Republican Party changed - and not in a good way.
The absolute hysteria in the GOP at the election of our first black president really demonstrated how racists had taken over. The fact that so many Republicans still believe that Obama is a Muslim and still believe he wasn't born in America demonstrates both that racism and the faith-based mindset of the GOP base. Evidence means nothing to them. They're simply going to believe what they want to believe, no matter what.
And that leads us to Donald Trump. (And to Ted Cruz as well, of course. Cruz is no better than Trump, not even slightly. It's just that Trump gets all the publicity, since he's so far in the lead.)
Back in Lincoln's time - and for years afterward - the Republican Party was progressive. But not anymore. Of course, it hasn't been progressive for a long, long time. Still, it wasn't particularly racist until Republican leaders saw a political advantage in racism.
For all its faults - and they're legion - the Democratic Party took the high road and did the right thing for America, even when they knew it would hurt them politically. The Republican Party deliberately took the wrong road, cynically taking advantage of racism for political advantage. Decades later, we've got the kind of Republican Party that strategy created.
They brought this on themselves. The crazy, angry, bigoted Republican base didn't happen by accident. That might not have been the original intent of their 'Southern strategy,' but that was the result - not just of the initial 'Southern strategy' but also of the continued use of racism, bigotry, and religious extremism since.