Lie down with dogs, get up with fleas, the old saying goes. If so, Donald Trump should be awfully itchy.

Trump has just augmented his ever-changing cast of mostly second-string campaign operatives with a new deputy campaign manager, conservative activist David Bossie. “A friend of mine for many years,” Trump told my Post colleague Robert Costa. “Solid. Smart. Loves politics, knows how to win.”

That’s one way to put it. Win at any cost would be another, and that’s being polite. If Bossie’s name doesn’t ring a bell, you’re lucky, because it means that you haven’t been immersed for the past two decades-plus in the mucky minutiae of the right’s no-holds-barred war against Bill and Hillary Clinton.

This is a war in which Bossie has risen from foot soldier to general, in large part thanks to his willingness to do anything in pursuit of his prey. He is the Captain Ahab of Clinton haters.

Some highlights:

Back in 1992, Bossie was working with Floyd Brown, of Willie Horton 1988 campaign ad fame, on an anti-Clinton effort that included a phone line in which callers could pay $4.99 to hear supposed sex tapes between Bill Clinton and Gennifer Flowers. President George H.W. Bush denounced the tactic as “the kind of sleaze that diminishes the political process” and filed a Federal Election Commission complaint against the group, the Presidential Victory Commission.

Bossie’s particular contribution to this effort involved harassing friends and family of a former law student of Clinton’s, Susan Coleman, who had committed suicide. As reported by CBS’s Eric Engberg, Bossie’s effort involved trying to prove that Coleman shot herself after having a sexual relationship with Clinton and becoming pregnant.

Bossie and another investigator pursued Coleman’s mother to an Army hospital where her husband was being treated for a stroke. “Here the two men burst into the sick man’s room, and began questioning the shaken mother about her daughter’s suicide,” Engberg reported.

Five years later, reporter Lloyd Grove recounted in The Post, “A chastened Bossie later told friends that the CBS story had made his grandmother cry.”

Chastened? Not so much. Grove was writing about Bossie because he had landed himself back in the news, this time as a committee investigator for then-Republican Rep. Dan Burton (of Vince Foster was murdered and shooting a pumpkin, or maybe a melon, to prove it fame).

After working with Bossie for several months on the investigation into Clinton’s campaign fundraising, the committee’s chief counsel, John Rowley III, and two other staffers resigned. Rowley issued a public letter denouncing Bossie’s “unrelenting, self-promoting actions.”

“Not since Roy Cohn — the bare-knuckled chief counsel for Sen. Joe McCarthy in the Red-hunting hearings of the 1950s — has a congressional staffer been so thoroughly demonized by his enemies,” Grove wrote. The comparison is particularly striking in retrospect because post-McCarthy Cohn became Trump’s lawyer.

Less than a year later, Burton was forced to apologize to his colleagues and Bossie resigned under pressure, after accusations that tapes of former Hillary Clinton law partner Webster Hubbell had been unfairly edited to exclude exculpatory comments about whether Clinton had known of his phony billing. (She had “no idea,” Hubbell said.)

In a closed-door Republican conference meeting, then-House Speaker Newt Gingrich told Burton he was “embarrassed . . . at the circus that went on at your committee,” The Post reported.

Since then, of course, Bossie, now at the helm of Citizens United, has continued his pursuit, now focused on Hillary. The Citizens United Supreme Court ruling grew out of his 2008 “Hillary: The Movie.”

My point is not that the Clintons are blameless — they aren’t — but that a candidate can be judged by the company he keeps and, especially, the individuals he hires. Trump has shed, sort of, Corey Lewandowski (recommended to him by, yes, Bossie) and Paul Manafort.

Now he has brought on Breitbart News Chairman Steve Bannon (recommended to him by, yes, Bossie) as the campaign’s chief executive, and with him questions about Bannon’s voter registration at a vacant Florida house and charges, ultimately dismissed, of domestic abuse by his then-wife.

The grown-up in the room with Trump — at least the one who’s not related to him — is campaign manager Kellyanne Conway, who has a reputation as a capable professional but has been praising her candidate for his supposed “pivot . . . to substance” and vowing, in regard to Clinton, “we’re going to fight her on substance.”

Uh-huh. Now comes Bossie, lauded by Conway as “a battle-tested warrior and a brilliant strategist.”

Scratch, scratch.

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