In 1868, Horatio Seymour ran for president as the nominee of the Democratic Party, or the “white man’s party,” as it was called. The Democratic heartland in those days was the “reconstructed” South. White men there loathed the Republican Party and its standard-bearer, Gen. Ulysses S. Grant — who relied on the votes of newly enfranchised blacks. Women, of course, could not vote.

Today, such overt racial appeals are as passe as the “solid” Democratic South. But racial polarization, alas, is not. As election analyst Sean Trende argues in a provocative new book, “The Lost Majority,” one plausible scenario for the American future is the entrenchment of “racialized” parties — a prospect that should concern anyone familiar with the American past.

We’re already well on the way. According to Trende, the Democratic share of the white vote for president has lagged its share of the total vote by a steadily increasing margin since 1980; the white electorate “leaned” against the Democrats by 10 points in 2008, even though Barack Obama did better with whites than John Kerry had four years earlier. A similar pattern holds for the congressional vote.

Data from other sources confirm these tendencies. More than three-fourths of African American voters identify with Democrats; Hispanics favor Democrats 47 percent to 24 percent, according to a 2011 Gallup poll. (The remainder in both groups call themselves independents.) By contrast, the Republican Party had an 11-point edge among whites in the Gallup survey.

Meanwhile, a July survey by the Pew Research Center showed an even more marked GOP advantage — 21 points — among white men. According to Pew, barely a third of white men consider themselves Democrats.

If the GOP is becoming the new “white man’s party,” the Democrats are reliant on women and people of color.

The causes are, by now, familiar: the white backlash against civil rights and the resulting long march of Southern whites from the Democratic Party to the GOP; the defection of white ethnic “Reagan Democrats” in the North; the GOP embrace of conservative positions on abortion and other social issues, which alienated many women; a Democratic policy agenda that favors not only affirmative action but also more spending on health care and education, areas in which large numbers of women and minorities are employed; the residential “big sort” into like-minded neighborhoods; race-conscious redistricting; the rise of a Latino population and the battle over illegal immigration.

The more relevant question is what, if anything, could halt the trend.

President Obama’s position as titular head of the Democratic Party may magnify the apparent “racialization” of the parties, with some whites rejecting him because of his race and some blacks and other minorities supporting him for the same reason. It’s notable that George W. Bush polled 11 percent of the African American vote and 44 percent of Latinos in 2004, compared with just 4 percent and 31 percent, respectively, for John McCain four years later.

The two-party share of the black and Latino vote may revert to more “normal” patterns depending on the race of the parties’ standard-bearers in 2016.

It is also possible that illegal immigration may fade as a wedge issue among white and Latino voters — because illegal immigration itself is waning. As economic conditions have improved in Mexico and stagnated in the United States, the net flow of illegals has reached roughly zero, according to demographer Jeffrey Passel of the Pew Hispanic Center.

But I doubt that a change in the skin color of the candidates would end the racialization of party loyalty; nor is there a demographic deus ex machina.

The trend toward a whiter, maler GOP and a browner, more female Democratic Party took root over decades in which broad structural changes — constitutional, economic and social — incentivized politicians to appeal to voters, openly and otherwise, in such a way as to produce today’s racializing parties. Those habits won’t change easily.

Still, it’s hard to see much future for a GOP that has minimal support from blacks and Latinos, especially when the latter are such a fast-growing part of the population. Though the growth of the Latino population appears to favor Democrats, the party can hardly afford to write off white men altogether.

An optimistic view is that party racialization is approaching the point of diminishing returns — and that the losing party in 2012 will conclude that it must broaden its base, or die.