I think this analysis of what we can expect from the debate by former Obama staffer Dan Pfeiffer is pretty good:
There are two ways to think about tonight’s massive audience. One one hand, 80 million people is a shitload of people. On the the other hand, 80 million people is less than half of the expected voter turnout this year. The handful of voters still deciding between Trump and Biden and the larger handful deciding between voting and not voting will disproportionately be in the half that doesn’t watch tonight. To the extent they know what happens, it will be through press coverage, online chatter, and viral clips. After Barack Obama lost the first debate to Mitt Romney in 2012, impressions of his performance got worse over time. In focus groups conducted the night of the debate, voters thought Obama underwhelmed. Within a few days, our polling showed voters thought the debate was an epic, potentially campaign ending disaster. This shift was not because people went back and watched the debate to reevaluate Obama’s showing. A narrative formed that Obama got his ass kicked (a narrative that was driven by some preemptive panic from liberal commentators).
That narrative was then consumed by the majority of the electorate that didn’t watch the debate via cable news, Twitter and Facebook. How the debate performances are framed, which moments the press focuses on, and what clips go viral will be more impactful on the campaign than what happens on the stage tonight. We all have the opportunity and responsibility to shape that narrative by pushing out Joe Biden’s best moments and pushing back on the misinformation that will go unchecked by the moderator.
This is important:
A decent amount of the voters and 100 percent of the reporters watching the debate will be doing so through a two screen experience. One eye on the TV and one eye on Twitter. Opinions about the performances of the candidates will be algorithmically affected. The moments that take off online early in the debate will be the most consequential because they will be shared the most and be seen repeatedly by people throughout the debate. Early reactions positive or negative will shape opinions of every subsequent moment. In an online world, failure and success tend to compound over time.
RT-thirsty reporters and pundits will be racing to make declarative statements about who won and lost. The earlier the declaration is made, the more attention said declaration will get. In 2012, Ben Smith of Buzzfeed declared Romney the winner of the first debate long before the debate was over. Knowing the Biden debate prep team as I do (they are best in the business), I am confident they are trying to front load some of his better moments for the first third of the debate.
[…]
The debate is here. There is nothing we can do about it so we might as well sit back, try to relax and watch for the moments that matter. One piece of advice: If you are a Biden supporter and you feel like channelling your panic into a tweet — don’t.
Develop an inner-monologue.
That last is good advice. If you’re panicking, please keep it all inside.