There was a time, not quite within the memory of Americans now alive, when the principal argument between the major parties was over the tariff. Democrats wanted a low one, Republicans a high one.

There were differences between the parties, but chiefly they were in perception. The Democrats were a more raffish crowd. They were said to enjoy themselves more. They were perhaps more foreign, sometimes talking in strange accents. On matters of patriotism, they were distrusted by Republicans, who had almost begun political life by “waving the bloody shirt” at Democrats for being sympathetic to the Confederate cause.

That last paragraph, portions of which I co-wrote with my reporting partner, James M. Perry, a quarter-century ago, once was true — but seems not so much quaint as antiquarian to our eyes today. Woodrow Wilson began the change, Franklin Roosevelt fueled it, Richard Nixon accelerated it and Ronald Reagan clinched the transition.

The charge that there’s “not a dime’s worth of difference” between the two parties — George C. Wallace’s proclamation in 1968 when he ran against both of them — now also seems antiquarian, though nothing about the Wallace of that period ever will seem quaint. Only from a European perspective, where it is sometimes argued that both Republicans and Democrats are hopelessly centrist captives of conventional mid-20th century capitalism, do these modern parties seem even remotely similar.

Biggest split since Civil War

Indeed, the two parties today are perhaps more different than they have been since the Civil War and the end of the reverberations over Reconstruction. They’ve swapped positions on the tariff — Democrats now want a high one and Republicans generally believe that higher tariffs mean lower productivity — and they differ about almost everything else.

One party generally believes in abortion rights, the other does not. One party embraces gay marriage, the other rejects it. One party would permit taxes to rise, the other would not. And though they both believe the nation is nearing a dangerous cliff on Social Security and Medicare, their prescriptions for avoiding the entitlements disaster are so different as to be irreconcilable. As we have seen.

They also differ in geography. The South is solid again, but today’s solid South is a deep Republican red. It used to be a Democratic blue — as blue and as seemingly permanent as the sky, even when viewed from the perspective of a pack of yellow dogs. The fact that almost none of the readers of this column ever has heard the phrase “yellow dog” in conversation seals my point.

This used to be a country of partisan contention but often of policy consensus. This was true as recently as 1960, when the two candidates, Vice President Nixon and Sen. John F. Kennedy, bickered a lot but differed little. Indeed, the only real disagreement anyone can remember from their classic 1960 debates was over two obscure islands, Quemoy and Matsu, not that many of us can remember precisely what the fight was about and how the candidates lined up. Except that Nixon vowed to be tougher.

In that year, Clinton Rossiter, the Cornell political historian, wrote that “there is and can be no real difference between the Democrats and the Republicans, because the unwritten laws of American politics demand that the parties overlap substantially in principle, policy, character, appeal and purpose — or cease to be parties with any hope of winning a national election.”

Rossiter died in 1970, as cracks in the world he knew were becoming evident. Nixon’s Southern Strategy left the yellow dog mortally ill and Reagan killed him. It was in large measure race, which had created the differences between the parties in the 19th century, that rendered the parties different as the last century ended and the new one deepened.

First the Democrats became the party of civil rights. Then the Republicans became the party of patriotism, small government and social conservatism. These were wedges that forced apart the foundations of the American party system. The parties that Rossiter believed couldn’t exist unless they mixed ideologies became parties that existed only because they separated ideologies.

Extremes prevail

Which brings us to Campaign 2012. A generation ago, it would have been inconceivable that one party would nominate an African-American and the other a Mormon — and that both would have Catholic running mates. We’re over that; all those things happened in one year. But a generation ago, it would have been possible to postulate a white Barack Obama type who resisted his party’s left wing and a Christian Mitt Romney type with moderate views — and the two of them having no serious ideological disagreements.

Not today. The real Obama is portrayed as an American Mitterrand, if not worse, and the real Romney could not survive in his original incarnation, a late 20th-century version of his father, whom he worshipped but at whose altar of accommodation and moderation he dares not linger today.

What we once saw as monumental choices — Nixon versus Kennedy, Nixon versus Hubert Humphrey — were contests with barely a difference. Hardly anyone alive thinks Nixon was as inspirational a figure as Kennedy, but hardly anyone can draw distinctions between them (both were Navy lieutenants in World War II, both entered the House in 1946, both moved to the Senate in the early 1950s) that remotely approach the differences between this year’s candidates.

Elections stand out

Indeed, the elections where the differences were starkly drawn — Lyndon Johnson versus Barry Goldwater in 1964 and Nixon against George McGovern in 1972 — stand out because they are so different in character from the rest of the contests. It is also not a coincidence that in both elections the candidate from his party’s extreme wing lost in a landslide.

We live in a different world, one in which the phrase “yellow dog” demands to be defined. So before we close, a primer on this curious canine.

There are several etymological explanations for the term, but two will suffice — the notion that a Southerner would vote for a yellow dog before he would vote for a Republican, or the companion idea that a Southerner would vote for a yellow dog if he were the nominee of the Democratic Party. The yellow dog is dead. Dead with it, alas, is the golden age of political metaphor. We may not all mourn the death of the former, but we can all grieve at the demise of the latter.