President Donald Trump speaks during a news conference on the...

President Donald Trump speaks during a news conference on the sidelines of the UN General Assembly in New York on Sept. 26, 2018. Credit: Bloomberg / Victor J. Blue

When politicians campaign for office, it’s common to find family members by their side. These days, office-seekers might be advised to watch their backs.

In Arizona, a campaign ad released recently features six men and women denouncing Republican Rep. Paul Gosar. One says the candidate “isn’t doing anything to help rural America.” Another: “Paul is absolutely not working for his district.” And: “He doesn’t have your interests at heart.” Only at the end do we learn that all six are siblings of the congressman, supporting his Democratic rival, David Brill.

For the record, Gosar’s mother says she’s firmly behind him. The congressman recently tweeted of his brothers and sisters, “like leftists everywhere, they put political ideology before family. Stalin would be proud.”

In rural Virginia last month, Bobby Goodlatte tweeted that he was donating the maximum ($2,700) to the Democrat seeking to fill the seat of a Republican congressman who had held it since 1993 — his father, House Judiciary Committee Chairman Bob Goodlatte.

In Wisconsin, a police officer, James Bryce, has appeared in an ad attacking the Democratic candidate running to replace retiring Republican Rep. Paul Ryan, the House speaker. “I don’t think people want to be represented by someone who’s shown contempt for those in law enforcement,” Bryce says.

He’s talking about his brother, Randy Bryce.

Wisconsin is also where the parents of Republican Senate candidate Kevin Nicholson each donated the maximum to the campaign of . . . incumbent Democrat Tammy Baldwin. His parents, Nicholson said, simply have “world views” that differ from his.

The first response here is to chalk these family fissures up to President Donald Trump, whose capacity to divide brother from sister and parent from child is now a staple of media coverage. Trump’s own White House “family” is not immune to feuding: Adviser Kellyanne Conway’s husband, George, regularly issues brutal tweets about the character and fitness of her boss. Last month, the uncle of another White House adviser, Stephen Miller, wrote a scathing piece for Politico, calling his nephew an “immigration hypocrite” for pushing draconian policies while ignoring his own family’s immigrant history.

But this is one case where Trump would have a persuasive defense: The political exploitation of family divisions long predates his administration. Nearly a century ago, in 1920, Theodore Roosevelt Jr., son of the late president, enthusiastically campaigned for Republican presidential candidate Warren Harding; Harding’s Democratic opponent was James Cox, whose running mate was Roosevelt’s cousin, Franklin. Theodore regarded FDR as a privileged dilettante: “He does not wear the brand of our family.”

Franklin returned the favor, with interest, in 1924, when Theodore ran for governor of New York against Al Smith. FDR denounced his cousin’s “wretched record” as assistant secretary of the Navy in the Harding administration. FDR’s wife, Eleanor, went him one better, touring New York State in a car with a giant papier-mache teapot — a not-so-subtle reference to the Teapot Dome scandal that had stained the Harding administration.

But for sheer family subversion, you can’t beat the campaign that unseated Minnesota Rep. Coya Knutson 60 years ago. The first woman elected to the House from Minnesota, Knutson had won in 1954 over the opposition of her Democratic-Farmer-Labor Party. Friction with party regulars brought payback during her 1958 reelection campaign. They recruited Knutson’s estranged husband, Andy, to write — or at least sign — a letter urging his wife to “go home and make a home for your husband and son . . . I’m sick and tired of having you run around with other men all the time and not your husband. I love you.”

The letter, which helped fuel a rumor that she was having an affair, went the 1958 version of viral; newspapers across the country reprinted it, often with the headline, “Coya, Come Home.” She lost reelection in a strongly Democratic year to a Republican opponent whose slogan was: “A Big Man for a Man-Sized Job.” (Note to political operatives: Do not try this tactic in 2018.)

As a political tactic, exploiting family splits is tempting. If candidates’ own flesh and blood won’t support them, why should voters? But it’s hard not to feel a pang of sympathy for candidates whose families turn against them. Campaigning is difficult enough — dealing with fundraising, 18-hour days and barrages of attack ads — without being shivved by a relative.

At least the knives come out only metaphorically these days — for centuries, politics was a genuinely bloody family business, going back at least as far as the Ptolemy dynasty in ancient Egypt. “It was rare,” historian Stacy Schiff once wrote of Cleopatra’s clan, “to find a member of the family who did not liquidate a relative or two.”

Compared with that, coping with a nasty tweet or an unkind video is a walk in the park — preferably accompanied by a trustworthy family member.

Jeff Greenfield is an author and veteran network news analyst. He wrote this for The Washington Post.