Instead, there was a sequence of external circumstances that thwarted Mr. Biden’s rivals and revived his campaign. There was the vote-counting fiasco in Iowa that denied Pete Buttigieg a moment of exultant triumph in primetime, leaving the former mayor without the sudden influx of cash and media attention that might have propelled him to victory in New Hampshire and allowed him to displace Mr. Biden as a centrist standard-bearer.
There was the debate in New Hampshire where Amy Klobuchar set Mr. Buttigieg back on his heels, frustrating his efforts to gain traction without quite achieving an upset win of her own. Had either of them defeated Mr. Sanders in New Hampshire, we could be looking at a transformed race right now; instead, they finished second and third.
There was Michael R. Bloomberg’s national onslaught of television ads that paralyzed most of the Democratic field in the biggest Super Tuesday primaries, pulling votes away from Mr. Biden but hoarding them for Mr. Bloomberg rather than one of his competitors in the early states.
And then there was Mr. Bloomberg’s calamitous debate performance in Las Vegas, where in the space of two hours Elizabeth Warren broke his dearly bought national momentum — loosening the former New York City mayor’s grip on voters who were favorably disposed toward Mr. Biden all along.
All the while, Mr. Biden slogged his way toward South Carolina, where for a combination of African-Americans and moderate white voters he was ultimately the only desirable option. And for many, he was a very desirable option indeed: If his numbers plunged after his humiliating losses in Iowa and New Hampshire, the former vice president never fell below the mid-teens in national polls and no other moderate candidate ever really overtook him. Mr. Biden hunkered down and waited for the map to turn in his favor, and it did.
That last-man-standing approach has given Mr. Biden his best shot in months to take control of the race and organize moderates into a wall of opposition against Mr. Sanders. Mr. Buttigieg’s withdrawal from the race on Sunday — another happy surprise for Mr. Biden — served as powerful confirmation that the basic dynamics of the campaign have changed.
But the strategy of patient endurance that worked for Mr. Biden in South Carolina may not allow him to best Mr. Sanders in a long campaign. And Mr. Biden can no longer count on a dynamic that has been one of his greatest assets in recent weeks — his rivals’ lack of interest in challenging him directly.