people think you need to run to the left in the primary, then pivot to the centre to win the general.

closer to the truth is the opposite - you need to run in the centre to win over the party base (which is more conservative than the general public), and then swing away from it in the general to mobilize independents and non-voters, which also means doing things intended to avoid alienating moderate republicans.

but, that means that it's really hard to beat strong, centrist candidates in the primary.

and, let's remember a point: obama did not run as a leftist. obama told us from the start that he was a conservative, ran as one and won as one - and governed as one. obama was the most conservative president of the last 50 years. the iraq war vote was a major issue, but his argument was not anti-war - he just thought iraq was a bad tactical decision made by a dumb commander-in-chief. he beat clinton by running to her right on social issues - he ran to her right on health care (she supported single payer at the time. he didn't.), he was broadly less supportive of queer issues (across the board), and he promised to cut spending and reign in the deficit. and, he had a penis. so, he was more appealing to the conservative, christian base of democratic primary voters than she was.

so, looking to 2008 as a model for biden's decline by pointing to these "progressive" candidates and suggesting they'll do what obama did, however left they actually are, is a completely false projection and a completely warped narrative (mostly told by very untrustworthy sources of information, all around). to start, biden was a lower tier candidate, at the time. i barely remember his campaign. the front-runner was clinton, you'll recall. the third candidate in closest contention was not biden, but john edwards. that was the guy that was supposed to win in the south, not biden. so, there was never a time in mid-2007 where biden was dominating the polls like this. he wasn't the vp in the last administration back then, either. nor was clinton seen as the dominant front-runner in 2007/2008 - everyone expected she'd be a given a run for it.

rather, what happened in 2007/2008 is that clinton ran on the left, and got beat by a strong centrist to her right. if history is to be repeated, this narrative would be brought out to explain the decline of bernie sanders.

and, if through some twist of history, a candidate like buttigieg holds on and gets a direct face-to-face with sanders in a field of two or three, we might have a tragedy on our hands.

if biden had anybody else his own age to compete with - gore. kerry. even clinton, again - they would probably beat him.

but, with an aging voting base, and a younger field, he's the last one left standing, and is going to get huge amounts of votes more or less by default.

and, it could be another 20 years before these voters finally die off.