Joe Biden walked onto another Democratic debate stage Tuesday night as the frontrunner and once against walked off the winner by default.

The moderators kicked things off by baiting Sen. Bernie Sanders and the former vice president into arguing about the Iraq War authorization vote in 2002. But even as Sanders stuck to his guns on this point, he wasn’t able to zoom out and explain what about Biden’s foreign policy record should make voters worried about his approach as president.

In the days before the debate, Sanders’s camp heavily telegraphed a big looming criticism of Biden’s past advocacy for Social Security cuts, but it didn’t happen.

Similarly, Sen. Elizabeth Warren’s camp indicated that she was finally interested in talking about bankruptcy, her main area of academic expertise and the subject of a years-long debate with Biden.

But that didn’t happen either. Instead, the biggest heat of the night came from a slightly odd Sanders-Warren disagreement over whether a 1990 election constituted something that happened “in the past 30 years.”

And it’s not just that the progressives didn’t really take on Biden; the moderates didn’t either. Sen. Amy Klobuchar of Minnesota reiterated her electability pitch, and it’s a pretty good one. But right now, electability-minded moderates are voting for Biden, and you can’t win them over without making a case against Biden any more than Sanders or Warren can beat Biden without really criticizing him.

To an extent, the issue is a tactical dilemma in a crowded field. It’s in everyone’s interest for someone to go after Biden, but it’s not necessarily in any particular candidate’s interest to be the one to do it.

Still, as Ian Sams, a member of California Sen. Kamala Harris’s erstwhile campaign, points out, her best moments in the polls came immediately after she fired hard shots at Biden in a debate.

One of the laziest takes in primary punditry is that Kamala's decision to engage VP Biden at the first debate "blew back" on her. There really is no data to support that claim, and the data we do have shows it was an unequivocal boon to her candidacy.



For example: pic.twitter.com/b1KWrrmF5K — Ian Sams (@IanSams) January 15, 2020

Harris’s problem was that despite making a great debate moment, she had no real policy point to make about school integration, and she didn’t follow up with any other lines of attack on Biden.

Julián Castro, similarly, did seem to suffer when he went after Biden, largely because the attack was unfair on the merits.

But Biden did back Bush on Iraq. He backed Social Security cuts. He backed a bad bankruptcy bill in 2005. And he lauded a bad budget deal with Republican Mitch McConnell as an example of sound bipartisan policymaking.

This pattern of behavior raises, to me, a real worry about a potential Biden presidency. Not that his talk of a post-election Republican Party “epiphany” is unrealistic — every candidate in the field is offering unrealistic plans for change — but that he has a taste for signing on to bad bargains. There’s potential for a critique of Biden that isn’t just about nitpicking the past or arguing about how ambitious Democrats should be in their legislative proposals, but about whether Biden would adequately hold the line when going toe-to-toe with congressional Republicans.

But once again, the candidates in second, third, fourth, and fifth place seemed largely content to argue among themselves rather than make the case against the frontrunner. With less than three weeks until voting starts, that’s how frontrunners win.