WASHINGTON -- The Democratic Party’s stampede to the presidency seems to be veering of late toward a racially charged contest.

Between Joe Biden’s admitted friendliness with segregationists four decades ago and California Sen. Kamala Harris’ recent attack on his anti-busing stance from that same era, the pivotal issue at this juncture seems to be race.

Let’s be clear: Re-litigating busing is a strategy, not an issue. Harris’ campaign apparently saw Biden’s distant history on race as a sure way to damage his candidacy as the only Democrat who can defeat President Trump.

All’s fair. Their gamble paid early returns.

Biden’s polling numbers dipped and Harris’ shot up, catapulting her out of the single digits and into second place in several surveys.

When a strategy works, you don’t change it, no matter how transparent it is. Thus, Harris seems committed to hammering Biden on race until he admits he was wrong. Both candidates had a chance to continue their fight from Iowa this past week. Speaking to reporters before a campaign event Thursday, Harris said, “It’s very important for us to be very clear about history.” And, “frankly, the vice president has yet to agree that his position on the kind of busing that took place when I was bused to school is wrong.”

Biden reiterated in an interview Friday with CNN that his record as a champion for equality is clear. He also admitted being surprised by her attack during last month’s debate. Hence, we may assume, his deer-in-headlights expression and the appearance of confusion, which had the dual effect of making him seem unprepared, vulnerable and out of touch in the generational sense.

At this stage of his career, Biden inarguably shouldn’t be surprised by anything. But Harris was shooting from two directions, and she knocked him off balance. It was a smart move, if not fully honest.

A person’s body of work should always take precedence over a long-ago decision based on the context of then, not now.

Meanwhile, if Biden isn’t careful, he may kill himself trying to prove his vitality. As temperatures hovered in the 80s in Independence, Iowa, on July 4, the 76-year-old jogged for several blocks of a parade, zigzagging across the street trying to shake as many hands as possible. Other candidates who also had shown up for the event -- New York City Mayor Bill de Blasio, 58, and former Texas Rep. Beto O’Rourke, 46 -- ambled along at a parade pace, perhaps in part because O’Rourke apparently had to leave his skateboard with airport security.

Despite Biden’s energetic performance, hecklers in the crowd were gratuitously cruel and disrespectful. One yelled out, “Where’s your walker?” Another approached and called him “Sleepy Joe,” Trump’s nickname for his principal foe. When Biden invited him to join him, the fellow declined.

Race popped up in other parts of the state, too. Pete Buttigieg was handed a viral opportunity at a July 4 barbecue in Carroll, Iowa, when someone in attendance, referencing the controversy over police violence in the Indiana town where Buttigieg is mayor, yelled out: “Just tell the black people of South Bend to stop committing crime and doing drugs.”

Cue cringe reflex.

“Sir,” Buttigieg began, “I think that racism is not going to help us get out of this problem … The fact that a black person is four times as likely as a white person to be incarcerated for the exact same crime is evidence of systemic racism, and with all due respect, sir, racism makes it harder for good police officers to do their job, too.”

Harris made clear in her debate attack that she doesn’t think Biden is a racist, but some of today’s younger voters might be hard-pressed to think otherwise -- and she surely knows it. Being accused of racial insensitivity can be a political death knell, needless to say. And, no matter how sincere Harris may be about her feelings about busing, from which she benefited as a child, the card was played for a political purpose. Likely, soon, we’ll see more candidates trying to establish their own racial justice bona fides.

Poor Biden, though holding first place, is still struggling to find his legs. As we register the importance of being on the right side of history, pray he doesn’t feel compelled to arm wrestle the youngest, Buttigieg, to prove his presidential strength. Besides, when it comes to political stamina, passion is an adrenaline trigger -- meaning Harris probably could beat them both.