Since 1932, blue-collar worker have been the bedrock upon which Democratic presidential candidates have built their coalitions. Franklin Roosevelt drew them into his party, and his successors, both winners and losers in the effort to win the White House, have put the votes and interests of blue-collar workers at the center of their campaign calculus and campaign rhetoric.

No more — which is why what happened last week in South Carolina made even the sharpest-eyed political expert squint.

If the political landscape seems out of focus, it is because there has been a fundamental shift in the topography of American civic life. You might even call it a tremor, if not an earthquake, rumbling through the nation as a result of this 21st-century development: More blue-collar workers today identify themselves as Republicans than as Democrats.

“This is a significant change, upending all of history from the Roosevelt years on,” says William Leuchtenburg, the University of North Carolina emeritus historian who, with more than half a dozen books on FDR to his credit, may be the leading expert on the 32nd president.

“The Great Depression was blamed on Republicans, big bankers and industrialists. At the same time, Roosevelt managed through his relief programs to sustain millions of Americans. The combination of those bound workers to the Democrats.”

That’s why the new blue-collar affinity for the Republicans is so jarring, but it’s based on compelling Wall Street Journal-NBC News survey data produced from interviews of 8,000 people, with a margin of error of a tiny 1.09 percent. Its implications are stunning, changing the way we look at the parties and the way the parties shape their messages, the way they recruit congressional and gubernatorial candidates, the way they behave on Capitol Hill.

Perhaps most startling of all: Poll figures show that as many Republicans as Democrats blame Wall Street bankers for the nation’s economic crisis.

All this explains why the Republican candidates, first in New Hampshire and most recently in South Carolina, have undertaken a searing and searching critique of capitalism, transforming all of our established beliefs that led us to assume that the Democrats were the party of blue-collar workers (and labor) and that the Republicans were the party of business (and capital).

The irony of this is that while the move of blue-collar voters toward the Republican Party began with Richard Nixon, who in 1968 cultivated voters who fell under the shorthand of “hard hats,” and accelerated with Ronald Reagan, who in 1980 deliberately sought votes from workers who became known as Reagan Democrats, the real change in the character of the Republican Party may have come during the House speakership of Newt Gingrich.

It was Gingrich who bid the GOP to look beyond its usual patrons on Wall Street and on the Business Roundtable and who tilled new political soil, creating a profile for himself, if not for the GOP, as being anti-government but not pro-business.

With his ties to the information-technology industries, which themselves swept away old assumptions of American commerce, and with a rhetoric of revolution, which was anathema to the stability-seeking zeitgeist of big business, Gingrich plotted a new path for Republicans. Just as the new entrepreneurs showed contempt for the staid, accommodationist world of the old Fortune 500, Gingrich showed contempt for the plodding, go-along attitude of the old GOP, personified by House Minority Leader Robert H. Michel of Illinois, whom he toppled.

Thus Gingrich, with greater affinity for the National Federation of Independent Business than for the U.S. Chamber of Commerce, may be the true engine of change in Washington and in the broader modern political culture.

This change occurred roughly during a period when the Democrats, under the leadership of House Majority Whip Tony Coelho of California in the 1980s and later under Bill Clinton in the 1990s, began a groundbreaking offensive to cultivate business groups (and seek contributions and support from commercial interests). These groups once were firmly in the GOP camp, so much so that White House insiders, and even the president himself, expressed surprise about the influence of the bond market on the Clinton administration.

It was a small leap from Gingrich’s positions in 1995 to his blistering criticism this winter of former Gov. Mitt Romney of Massachusetts as a corporate viper who put profits ahead of people, just the sort of phrase that used to tumble effortlessly from the lips of politicians who opposed Gingrich’s party.

Now, Barack Obama is putting together a campaign effort that all but writes off the greatest legacy of a president he reveres, Franklin Roosevelt. It is not so much President Obama’s temperament that veers him away from blue-color voters as his reading of the political landscape — and perhaps his political circumstances.

“We know that blue-collar workers have been especially hard hit and that definitely affects their views,” says Adam Seth Levine, a Cornell political scientist. “Even in divided government, Americans blame the president and his party for the bad economy. That makes them more likely to identify as Republicans.”

As a result, Obama is assembling a coalition that doesn’t depend on the voters that were the mainstays of the presidential coalitions of Roosevelt, Truman, Kennedy, Johnson, Carter and even Clinton — though it is telling that each president in that string was less committed to the old formula than was his predecessor.

Political coalitions change over time, rendering them almost unrecognizable from century to century. The 19th-century Democrats, with strong religious-conservative elements, opposed many of the principles now associated with the party, especially a strong central government and civil rights. The Republicans, with a strong elitist and reformist tint, advocated a strong federal government and openness to rights for newly emancipated slaves.

Now the parties — always changing, more often being led than leading — seem to be etching new profiles.

The old chestnut of Leuchtenburg’s lectures, that you always could count on blue-collar workers to be Democrats, is no longer true, just as the reason for that iron rule of politics — the vast mass of unionized voters in auto plants steel and textile mills — has faded. Parties in and out of office campaign for change, but the biggest change of all sometimes comes within the parties themselves. That’s the biggest story of Campaign 2012 thus far.