More often than not it is deliberately manipulated for political expediency. Either side can do it, and both sides have.
It used to be that Republicans were the party of Abolition, and Democrats the Pro-Slavery faction. This dichotomy was the result of economic and historical conditions, not philosophical positions. During the harsh Reconstruction after the Civil War, the Southern Democrats came to resent the Northern Republicans who they correctly perceived as the source of occupation, oppression and the Carpetbagger. This helped solidify racism as a truly toxic psychosocial pathology having little to do with political or economic issues. It led directly to the KKK and Jim Crow, black voter suppression, segregation and all the rest. Black folks, for their part, tended to vote Republican.
What I personally witnessed growing up in the Deep South (with the helpful perspective of a Latin not culturally saturated by, and therefore realistically aware of, local prejudices) was a reversal of this traditional division. America was undergoing the Civil Rights movement, and the GOP's Southern Strategy was to systematically exploit this ancient regional divide and Southern Democratic fears for political advantage.
http://en.wikipedia.org/wiki/Southern_strategy
The strategy was first adopted under future Republican President Richard Nixon and Republican Senator Barry Goldwater[6][7] in the late 1960s.[8] The strategy was successful in winning 5 formerly Confederate states in both the 1964 and 1968 presidential elections. It contributed to the electoral realignment of some Southern states to the Republican Party, but at the expense of losing more than 90 percent of black voters to the Democratic Party. As the twentieth century came to a close, the Republican Party began attempting to appeal to black voters again, though with little success.[8]
In 2005, Republican National Committee chairman Ken Mehlman formally apologized to the NAACP for ignoring the black vote and exploiting racial conflicts.[9][10]
Since then, conservative apologists and revisionists have vainly attempted to rewrite history and dissociate their party from this strategy, (perhaps because they themselves are not personally racist?) but it was real and it was obvious at the time. I saw it, I knew what was happening while I saw it happening around me, before it even had a name. I was there.
It is this personal experience that has led to my remarks on this topic. It is not that Conservative principles lead inevitably to racism. That is nonsense, after all, there are black Conservatives.
But the truth remains, the modern Republican Party continues to follow this policy, not only in regards to racism, but as it applies to other social and cultural pathologies that characterize the constituencies they have come to increasingly rely on for votes. This includes religious fundamentalism, extreme nationalism, nativism, militarism, anti-science attitudes, conspiracy theories, irrational suspicion and downright hatred of women, homosexuals, intellectuals, academics and the press.
You've seen it happen yourself; some racist bigot stands up in a town hall (or even in Congress) spouting venomous birther nonsense and the Republican candidate (who may not be a racist or extremist himself) simply winks and nods and makes no attempt to correct or chastise the rabble-rouser. As far as I know, only one Republican (Sen John McCain) made the effort to put one of these redneck yahoos in their place, and it only earned him the hatred and mistrust of his own party.
This is pandering to the worst aspects and sickness in the American psyche for a few votes, and the Republican Party will pay a bitter price for this perverse sycophancy. It has unleashed a monster, created a Frankenstein, and events now in Congress and on the street testify that this beast will not go away after it has helped the Right achieve political domination. They will turn on you, as a matter of fact, they already have. The GOP can no longer control its own Brownshirts. Welcome to Weimar Amerika, RINO.