The tragic events in Charleston this month have released years of racial and political tension in the South, and the pressure is being felt by Republican officeholders across the region.

Why the Republicans? Because it is increasingly difficult to find officeholders in the region who are not Republicans.

The South was once home to the "yellow dog Democrat" who would vote for a mutt over someone from the party of Abraham Lincoln. Now, the party of the Great Emancipator has made Dixie its bedrock, the base of its Electoral College vote and its majorities in Congress. Many a great-granddaddy buried in rebel gray has been rolling over in his grave for some years now.

The South's rejection of its Democratic DNA began more than 60 years ago with a Supreme Court decision, and significant historic milestones have followed like clockwork in almost every decade since. (The one exception was the 1970s, when Watergate torpedoed the Nixon presidency and led to the election of Jimmy Carter in 1976.)

The late Nelson Polsby, an influential and at times contrarian political scientist, wrote a book arguing that it was air conditioning that made the South competitive. It brought Republicans from other parts of the country into the South — as retirees and as employers — in growing numbers after World War II.

While Polsby's observation clearly applies in states such as Florida and Georgia, many other observers have tended to attribute the Southern shift to Republicanism to changing party alignments on issues. Salient among these are views on race, civil rights and federal power.

Many Southerners have stood by traditional values on social issues as well — including guns, school prayer, abortion and same-sex marriage. And these voters have found their conservative stands more welcome in the Republican Party than in the Democratic.

Here are a few of the major milestones in the migration of these Southern voters.

1954 The Supreme Court ruled in Brown v. Board of Education that "separate but equal" schools were inherently unequal. This became the basis for a school integration effort still bearing fruit and causing conflict today. In parts of the South, "massive resistance" to Brown strained state and federal relations much as the anti-slavery movement had done in the mid-1800s.

1964 The Civil Rights Act was passed despite a months-long filibuster by Southern Democrats. The filibuster was broken by the rest of the Democrats in the Senate in league with most of the chamber's Republicans — but not all. Six Republicans voted with the Dixiecrats, and one was Barry Goldwater of Arizona, who was that year's GOP nominee for president. One of the filibuster leaders was Strom Thurmond of South Carolina, who switched his party allegiance to Republican and backed Goldwater for president. President Lyndon Johnson was elected in a landslide that November, but Goldwater carried Thurmond's home state and its Deep South neighbors: Georgia, Alabama, Mississippi and Louisiana. It was a harbinger of things to come, when these states would help flip all the "Solid South" from D to R in the Electoral College.

1980 Jimmy Carter, a former governor of Georgia with a distinctly Southern accent and Baptist background made Southerners feel comfortable in their traditional home party again — but only temporarily. In 1980, Ronald Reagan's presidential nomination would pull the South across the line for good. He had won the GOP nomination by sweeping the primaries and caucuses in the South, beginning with the South Carolina primary. He then wrested the South away from Carter, who won only his home state of Georgia.

1984 Ronald Reagan won 49 states and carried in the biggest group of Southern Republicans in Congress since Reconstruction. In 1984, Reagan's massive win extended his party's new hegemony in the region beyond Congress to the state legislatures and even some local jurisdictions – the "courthouse politics" base of Democrats' power for a century.

1994 After another Democrat from the South, Bill Clinton, had won the White House two years earlier, the unpopularity of his views on guns and government contributed to broad-scale blowback in his first midterm election. It was called the "Gingrich Revolution" for the Georgia congressman who became the first Republican Speaker of the House in 40 years. But it went beyond the House. Republicans also stormed back into the majority in the Senate and captured a majority among governors. Leading the way was a massive surge in the South, where most members of Congress and governors had been Democrats throughout the century since Reconstruction. From 1994 on, the majority in all three categories would be Republicans.

2000 Both parties nominated Southerners again, with Texas Gov. George W. Bush opposed by Tennessee Sen. Al Gore. But the South had no trouble deciding. All 11 secessionary states went for Bush, as did the adjacent "border states" of Oklahoma, Missouri, Kentucky and West Virginia. Overall they provided more than half Bush's 271 Electoral College vote. The rest of the country went for Gore by a margin of 2-1. Had Gore won his home state, or any other Southern state, he would have been elected. Four years later, Bush would be re-elected in the same manner, with well over half his Electoral College vote coming from the South and two-thirds of the rest of the country's Electoral College vote going to the Democratic nominee, Sen. John Kerry of Massachusetts. The Southern bloc had become populous and cohesive enough to counter-balance even such a pronounced preference for the Democrat in the rest of the country.

2008 Barack Obama, the first non-Southern Democratic president since John F. Kennedy, seemed to hit the pause button on the Republicanizing of the South. It was stunning to see the first African-American nominated by a major political party carry three formerly-Confederate states in Virginia, North Carolina and Florida. He won Virginia and Florida again in his re-election four years later. He did well in the South both times largely by the same means he won nationally, mobilizing voters who were younger, women, Hispanic and African-American. But many of these less-frequent voters did not turn out for the midterm elections that would follow.

2010 In his first midterm as president, Barack Obama saw the GOP sweep the significant elections in the South, propelling them to a historic takeover win in the U.S. House. Significantly, Republicans achieved the highest degree of control in state governments since the 1920s, enabling them to draw district maps at the state and federal level that maximize the efficiency of GOP votes. This is relatively easy to do, in the South as elsewhere, as the greatest concentrations of Democratic votes occur in urban areas while Republican voters are more widely distributed geographically. These maps have made the GOP's legislative majorities notably secure through 2020.

2014 In his second midterm, Obama saw yet another GOP resurgence in the South helping the GOP take over the majority in the Senate and increase its margins in the House. The Republican march also continued in state legislatures. As in 2010, the electorate trended older, whiter, more male and more affluent. As a result, Republicans have become the all-but-permanent residents of the statehouses and masters of state legislative majorities throughout the region.

This includes the 11 states of the secession and to a lesser degree the four border states, which among them still have two Democratic senators and three Democratic governors.

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