When I was younger, I had an idea for a satire in which a group of rogue genealogists would get blood samples from racially incendiary white politicians. They’d run DNA tests on them to see if they were part black, then publish the information on those who were.

This was way back before you could order DNA tests online and watch television shows about celebrities seeing their results. I don’t write satire, and wisely abandoned the idea. But the broader concept of bloodlines and our construction of political identity popped into my head during a recent discussion about the election.

Donald Trump’s supporters, 90 percent of whom are white, have measurably less positive feelings about African-Americans and several other groups than other Republicans or the public at large. But how many of his voters are “black,” not by our current categorization but under the “one-drop rule” saying that no matter how white they looked, anyone with a drop of black blood was categorized as such? Variations of the one-drop rule persisted in law until 1983 in Louisiana, after a mixed-race woman unsuccessfully sued to become legally white, and legislators overturned the law in response to the fractious case. Some argue that the one-drop rule had already been eroded by the 1967 Loving v. Virginia Supreme Court case on interracial marriage, now the subject of a new movie.

Whether we are talking about 1967 or 1983, the idea of blackness as something that is immutable despite skin color, hair texture or other phenotypic indicators is not just an antebellum relic. It persisted well into the modern era and the lifetimes of many of us.

In a country where there are now more Latinos than blacks, and American racial and ethnic diversity ranges far beyond a black-white axis, blackness still has specific political and economic significance. Entire bodies of law — voting law, housing law, marriage law, property law, among others — were constructed around the exclusion of blacks from enfranchisement and wealth building. When a federal judge this week described as “insane” a system currently purging voters from the rolls in North Carolina — a Southern swing state that went for Obama in 2008 and Romney in 2012 — she went on to say that it hearkened back to 1901 and the Jim Crow era when black men technically had the right to vote but found it almost impossible to exercise. The legal legacy of the suppression of black votes echoes today.

So how does this relate to DNA tests and bloodlines? Well, according to a 2014 study of 145,000 DNA test results by the firm 23andMe, 3.5 percent of people who consider themselves white Americans are actually part black.

That’s approximately 6.9 million non-Hispanic white Americans with black blood. Although the sample size is generous, it’s also self-selecting based on who chose to take or bought the test, which is likely skewed toward Americans with higher disposable income.

The U.S. Census Bureau has changed how it tracks race in many ways since the first census was conducted in 1790. Through 1950, the census taker was the ultimate arbiter of what race was marked on the form. (One of my great-great-grandfathers was marked as black on one census and “mulatto” or mixed race on a subsequent one.) In 1960, the census began allowing people to choose their own race, but they could choose only one. And in 2000, it began allowing respondents to choose more than one race and/or the category “some other race,” plus, if they chose, the ethnicity Hispanic. Of the non-Hispanic population, 0.3 percent marked only “some other race,” while 2.8 percent marked two or more races.

There’s no one running around with a ledger demanding that these black-descended white Americans submit to the former one-drop rule. But if everyone magically knew their results before Election Day, would it shape how some people saw their own identity and their relationship to our political system? If you were “all white” but you found out your spouse was what Louisiana courts in the 1980s called “colored,” how would that affect your construction of self and political decisionmaking? Would it change your stakes in this election, or spur efforts to conceal the information, as people actively passing for white did generations ago?

Southern whites are by far the most likely to have black blood, as roughly 10 percent of whites in the region do, including in the swing states of Florida and North Carolina. The possible reasons for that density include the early history of America when blacks and Irish were indentured servants and sometimes had consensual relationships and families; infrequent intermarriages before the abolition of slavery between free blacks and whites; the rape of black women by white men during and after slavery; and the efforts of white-looking mixed-race Americans to “pass” for white to gain freedom or economic and legal advantage. In fact, via his white American mother, President Obama is related to a black man, John Punch, enslaved in the 1640s. Punch’s mixed-race children produced descendents who passed into whiteness.

Over time, many descendants of Americans who passed for white were no longer aware of their racial heritage, whether they would have been proud of that fact, neutral or mortified, as was a prominent white supremacist who found out on television in 2013 that his DNA test results included 14 percent sub-Saharan African heritage.

As we look not only to Election Day but beyond, we have a chance to see how our constructions of racial and political identity fit into the broader sweep of history in ways that may challenge our assumptions.