The results from the second round of Democratic presidential primaries this week didn't bring an official end to the race for the party's nomination. But the massive setbacks for Sen. Bernie Sanders (I-Vt.) might just as well have ended his hopes for a first-ballot win, or even holding former Vice President Joe Biden off from winning a first-ballot majority himself. Despite having won Michigan four years ago against Hillary Clinton, Sanders lost every county in the key Midwestern state this week — and every county in Missouri and Mississippi, as well.

Even more ominously, the race's final contest in Washington state turned out too close to call by the next afternoon. In 2016, Sanders got 72 percent of the vote in the progressive-friendly state and the lion's share of its delegates. By the end of the count, Biden might have a lead of nearly 200 delegates — perhaps insurmountable in Democrats' proportional-allocation primary system.

All of this prompted calls for Sanders to bring the primaries to an end. At least for now, Sanders disagrees. Sanders announced his intention to keep on fighting, and pledged to make the upcoming debate on Sunday in Arizona — the first one-on-one debate of this cycle — to press Biden on policy and values issues. If nothing else, this gives him one last opportunity to force the primary back to ideology and policy — or, more likely, to demonstrate Biden's shortcomings as a frontrunner and to reverse the electability argument. Could Sanders still forge a potential path to victory?

The Biden tsunami of the past week has been stunning. Before Biden won in South Carolina — his first-ever presidential primary win — the question appeared to be whether the Democratic establishment could prevent Sanders from using a split field to gain enough delegates to win on a first ballot. Sanders had either won or virtually tied in the first three contests of the season (Iowa, New Hampshire, and Nevada) while Biden had finished largely out of contention in all three.

This raises a key question, both for Sunday's debate and for the general election. Did Biden stage an impressive and historic comeback to prove his electoral mettle? Did Sanders suffer a historic collapse instead? Or did everyone misread the 2016 electoral cycle as a populist uprising, when in fact it might have been a rejection of one specific person across the partisan divide: Hillary Clinton? Jonathan Chait argued that the big lesson of the last electoral cycle wasn't that socialism was on the rise, but that "large portions of the public, even of the Democratic electorate, simply detested her." Without Clinton as the sole opponent, Sanders simply doesn't have enough of a constituency within the Democratic Party to compete.

That may well be one factor, and it would account for Sanders' poor showing the last two weeks. It still fails to explain why Biden nearly collapsed out of the race against such a weak candidate in the first place, though. Biden came into the primary with plenty of structural advantages — eight years as the vice president to the highly popular President Barack Obama, strong connections to the institutional donor base, and some claim to a track record of appealing to traditional Democratic constituencies in the "blue wall" states. For months, his opponents competed mostly for credibility among progressive voters while Biden focused on moderates and African Americans.

And yet, Biden ended up performing so weakly that it encouraged late entries by Deval Patrick and Michael Bloomberg to rescue the party from Sanders. Patrick turned out to be a non-factor, but Bloomberg's entry started a stampede to the center among other candidates in the race, too. It also gave Sanders and Sen. Elizabeth Warren (D-Mass.) a handy target in debates that largely shielded Biden from attack — and it still didn't pay off until South Carolina.

The quality of Biden's sudden good fortune raises another question about his viability, too. Until Sanders started offering arguments about the good side of Fidel Castro and communist China, Biden had trailed in a number of Super Tuesday states. After winning South Carolina and having the remaining center-lane candidates drop out, Biden began winning in places he'd barely visited — Minnesota, Massachusetts, Maine, and arguably Texas — where Sanders had been presumed to be ahead. Polls suggested that Sanders would score a huge delegate advantage over Biden in progressive California, but Biden finished within seven points of Sanders and with just 50 fewer delegates.

The impression this leaves is of a candidate who is winning by default, not by skill or argument. Exit polls favoring electability over policy amplify that impression. Sanders must believe that to be true, too, which is why he's defying the delegate math for a few more days to take a direct run at Biden on stage. All Biden has to do at Sunday's debate is survive it without doing too much damage to his own case. Sanders can only win now if Biden stumbles so badly as to make himself the more dangerous choice in a general election, and it will take a historic stumble to make the Fidel apologist the safe choice.

However, Biden's performance thus far should raise serious concerns about his ability to compete against President Trump. It's true that Trump largely won four years ago because of the incompetence of Hillary Clinton's campaign and the disgust she inspired among voters, as Chait notes. This time around, though, Trump's record primary turnout while running largely unopposed hints at the kind of voter-turnout organization that he largely eschewed in 2016. If Sunday's debate is Bernie's last stand, it may also be the last chance Biden has to prove he can actually compete on his own terms in a presidential contest. A default candidacy may not suffice in the general election.

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