What is the most reliable way to destroy a political career? Financial shenanigans, criminal records or college antics are all reliable showstoppers, but it's usually the salacious sex scandal that brings the house down. Jack Ryan, who ran for the Senate against Barack Obama (for a while), brought us Parisian sex clubs. Mark Sanford, former governor of South Carolina, famously hiked the Appalachian Trail. And former senator John Edwards offered a scorching mess of "What To Expect When You're Expecting."

Add race to the question - particularly interrace - and political prurience goes into overdrive. The confluence of miscegenation and politics speaks to America's fundamental anxiety about racial boundaries. It's been a rug-puller of careers as long America has been a republic.

When the candidate is one race, and the spouse/partner/"friend" is another, opponents find a combustible cocktail to stir voter insecurities. Ask the ghost of Thomas Jefferson, who weathered decades of criticism about his relationship with "Dusky Sally," his mixed-race slave who bore six mixed-race children. Consider Richard Johnson, vice president under Martin Van Buren, whom the press condemned for taking a "jet-black, thick-lipped, odiferous negro wench" as his common-law wife. Fast forward to Harold Ford Jr., who was maligned during his 2006 Senate campaign in Tennessee as a white woman-loving playboy. For these figures - just a few of many - the color line drew rings around their reputation.

Why would an interracial relationship become a dangerous political liaison? For most people, sex and relationships are private actions, but for public figures, intimate life turns into news. Add race to the mix, and it raises eyebrows. Obama had a white girlfriend in college? Sarah Palin may or may not have dated a black athlete? There are European royals of black and Asian descent? (Lichtenstein and Denmark.) At minimum, such pairings are imaginatively interesting. But why does it matter?

Theoretically, there is no legal barrier. In 1967, in Loving v. Virginia, the Supreme Court made interracial marriage legally available in all states. But a transformative case does not automatically transform personal opinion. Some jurisdictions did not comply with the ruling. By popular vote, South Carolina and Alabama were the last states to overturn their prohibitions in 1998 and 2000. Even though any such law would be unenforceable, 40% of Alabamians voted to retain the statute in the state constitution.

Opinions of interracial relationships are like opinions about Marmite or cruises: they are extremely partisan and rarely mild. For some, racial intermixture is difficult, confusing, or offensive. For others, interracial relationships perfectly demonstrate the irrelevance of race.

Collectively, Americans profess a love of colorblindness. We point to our multiracial president as proof that race barriers can be broken. We also parrot Martin Luther King's idea that it's not what's on the outside that matters. But a close examination of the common "race doesn't matter" statement reveals avoidance and discomfort. This diagnosis usually depends on one of two symptoms: Vision Deficiency (they don't "see color") or Rainbow Delusion (not caring if a person is "black, white, purple or green"). It's an impossible conundrum: how can race simultaneously be an irrelevance and an obsession? Unsurprisingly, these optical defects are rapidly cured in neighborhood selection, school choice and intimate relations.

Freedom of association, on an intimate level, doesn't apply for political figures. When it comes to exposing candidate faults, race is a familiar boost of indignation. Former Senator Bob Bennett, a Republican from Utah, understands this. In 1999, he publicly identified the two things that might have prevented George W. Bush's nomination on the Republican ticket. The only obstacles, Bennett declared, would be if Bush "steps in front of a bus or some woman comes forward, let's say some black woman, with an illegitimate child that he fathered."

Bennett's obtuse expression merits a lifetime subscription to Essence magazine. He got it ridiculously wrong by blaming black women and public transportation for the downfall of public figures. But he also got it so perversely right by articulating the persistent and antiquated fears that have long been, legally speaking, void.

Miscegenation is the original race card. Accusations have affected all political persuasions and races, to a point where the fixation becomes the candidate's defining element. Jefferson is certainly not alone in the accusations against him. Abraham Lincoln's opponents published a campaign cartoon, "The Miscegenation Ball," that lampooned an interracial regime where white men and black women freely dance, flirt and carouse. And Strom Thurmond, who infamously denounced integration of homes, schools and pools, was ultimately revealed to have a mixed pool of his own.

The interracial card also affects nonscandalous and nonsecret relationships in politics, even as interracial marriage has increased almost 30% in the past 10 years. Very few public figures in American history have married a person of another race, which speaks to a curious relationship between race mixture and electability. The interracial marriages of Senator Edward Brooke, Senator Phil Gramm and Gov. Nikki Haley are atypical. While interracial marriage remains relatively rare, at 8.4% of all marriages, in the political realm it is practically shunned.

Lots of private issues about candidates have political consequences: college extracurriculars (Clinton's inhalation), recreation choices (Kerry's windsurfing) and physical appearance (Dukakis' eyebrows). And this includes the identity of one's partners.

The preoccupation with difference completely disregards the interpersonal relationship itself. It's the polar opposite of colorblindness, and the prism reveals only color and not character. In my book on interracial marriage, Prof. Camille Nelson, dean of Suffolk Law School, relates a personal account as a black woman married to a white man. While she and her husband were enjoying a wedding anniversary trip to a Caribbean resort, they were approached by an American tourist, who asked the conservatively dressed couple sitting in the piano lounge, "Are you two porn stars?" In the mind of the stranger, their pairing was so tinged by race - blocking any possibility of a real relationship - that he only saw sex.

In another narrative, Prof. Bennett Capers, a black man married to a white man, received innumerable stares during a visit to his Southern hometown. The couple's presence confused locals not because they were gay, but because they were two men of different races having dinner together. Integrated socializing was that rare.

Sometimes, interracial marriage may have political benefits. While running for Public Advocate in New York City, Bill de Blasio placed his interracial marriage at the center of his campaign. In Utah, Mia Love, who is Haitian-American and Mormon, is running for Congress as a Republican, lovingly supported by her husband Jason, who is also Mormon. As family life is a standard part of a candidate's narrative, some public figures have successfully transcended racial scrutiny to demonstrate assimilation and membership within a community.

Aren't these success stories proof that de jure discrimination is a thing of the past? Shouldn't we be thinking of the substance of character rather than rehashing the vagaries of color? Aren't we post-racial yet?

In America, where elections are often a referendum on the character of a candidate, we vote not only for individuals, but also for the entire landscapes in which they live and love. We consider their ideologies, their personality, their pasts and their relationships. Our nation has liberally appropriated the mantra that race is less important than the content of a candidate's character. To realize its true intention without weakening its mythical force, it must also extend to the color of their company's skin.

Kevin Noble Maillard is a professor of law at Syracuse University. He is the co-editor of "Loving v. Virginia in a Post Racial World: Rethinking Race, Sex, and Marriage."