Montana Gov. Steve Bullock (D) in Des Moines in August 2018. (Charlie Neibergall/AP)

Montana Gov. Steve Bullock is the latest entrant in the now 22-person Democratic presidential candidate field. While he touts his ability to win a state that President Trump carried by 20 points, that’s not likely to be persuasive enough to the voters to fuel Bullock’s rise. Instead, he is competing with a few others in the hidden path to the nomination: the “Biden slips up” lane.

Bullock is one of a few Democrats whose essential argument is that they are liberal enough while still being acceptable to swing voters. Rep. Seth Moulton (Mass.), for example, recently argued that Medicare-for-all is a bad idea because it would create a medical version of the Post Office. Former Colorado governor John Hickenlooper declared that he is running to ”save capitalism,” while Colorado Sen. Michael F. Bennet touts his “bipartisan work” in the Senate. Sen. Amy Klobuchar (Minn.) also focuses on her track record of winning blue-collar votes in rural areas and small towns in the Midwest while distancing herself from Medicare-for-all and providing college for free.

The problem each of these candidates face is that former vice president Joe Biden stands in their way. Polls show that nearly half of Democratic voters say they are moderate, and these people are the ones who presumably would be most receptive to the pack’s “liberal enough” appeal. But this group is Biden’s base. The same polls show Biden dominating among moderate voters, receiving between 38 percent to 58 percent of their support. None of these candidates have a chance as long as Biden retains this backing.

This places Bullock and the others in a bind. They can’t attack Biden directly because such an attack could backfire. Experienced campaigners know that attacks hurt both the target and the accuser; in a 22-candidate primary, the person who fires first can bring down the big game only to see the fruits of their success go to a competitor. So these candidates have to wait and hope that Biden falls — either on his own or by another’s hand.

Fortunately for them, there’s a decent chance that could happen. More progressive candidates have every reason to go after Biden, as their voters already prefer someone more aggressive and uncompromising. Candidates such as Sens. Bernie Sanders (I-Vt.), Elizabeth Warren (D-Mass.) and Kamala D. Harris (D-Calif.) are all competing to be the progressive champion, and would benefit from being seen as the person best able to take Biden down. Responding to their attacks would be tricky for him. He can’t reject the party’s progressive consensus, but he also can’t go too far in embracing it. Navigating that precise course would be a difficult task for the notoriously imprecise former vice president.

Biden could also stumble of his own accord. He is known as a walking gaffe machine. He’s never been a front-runner before and could easily wilt under the nonstop glare of attention. As a candidate, Trump survived a series of controversial statements during the Republican primaries that would have sunk anyone else, but he had the fervent backing of perhaps a quarter of the voters who simply did not find any establishment politician credible. Biden neither inspires that level of passion nor has that cult-like persona. It’s not difficult to imagine moderate Democrats turning to someone else if he stumbles.

This means that one of these candidates could easily become this cycle’s breakout star, like John Edwards in 2004 or Rick Santorum in 2012 for the GOP. Both Edwards and Santorum labored for a time prior to primary season in relative obscurity, rarely if ever reaching double digits in the polls. Yet when candidates in front of them collapsed, as Democrat Richard Gephardt did in 2004 and a host of conservative hopefuls did in 2012, the voters searched for an alternative instead of backing the front-runner.

That’s when the patient, unnoticed work from Edwards and Santorum paid off. They both offered a new face that seemed better than the old one, rocketing to stardom in the month before the Iowa caucuses. Bullock, Klobuchar and their fellow moderates are all hoping to be this cycle’s version of these candidates, racing to the front after the pacesetter wrecks his campaign.

Neither Edwards nor Santorum won the nomination, but neither were trying to grab votes from their party’s front-runner, either. Santorum’s rise left the less conservative leader, Mitt Romney, untouched. And Edwards similarly couldn’t amass enough voters to knock down John F. Kerry. Should Biden collapse, anyone who can co-opt his much larger backing will be in a much stronger position than Edwards or Santorum ever were.

Conventional wisdom says these less progressive, later entrants don’t have a prayer. But don’t place too much stock in that wisdom. The “Biden falters” crowd knows that front-runners can crash and burn, and each of these candidates is waiting behind the main pack for that to happen. If it does, don’t be shocked when one of them climbs rapidly on the leader board.

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