Biden’s Balancing Act

Similarly, on health care, Mr. Biden has spoken often of the part he played in helping pass the Affordable Care Act, even though many Americans continue to see it as flawed: Just 42 percent of registered voters nationwide — including only 37 percent of independents — said the law had been a good idea, according to an NBC News/Wall Street Journal poll released in March.

But the “Medicare for all” proposal pushed by Mr. Sanders and other progressives was no more popular, with only 43 percent of voters supporting it, according to the NBC/Journal poll. Mr. Biden’s proposal, adding a Medicare-like public option to Obamacare, was far more popular, approved of by nearly three in four voters in the poll.

When it comes to racial justice, Mr. Biden also has a delicate line to walk. He is seeking to harness the momentum of a racial-justice movement that has drawn an uncommonly strong wave of popular support — but he has shied away from embracing some of its most trenchant elements. While he supports the sweeping police reform bill passed by the House, he has resisted calls to “defund the police.”

Notably absent from most Democrats’ convention speeches this week were mentions of the clashes, in cities such as Portland, Ore., between protesters and federal agents ordered in by Mr. Trump. Polling has shown Americans roughly split on whether Mr. Trump was right to send in those officers.

It’s an open question whether Mr. Biden gives voice to the outrage of many liberals who see Mr. Trump’s actions in Portland as a gross abuse of power, or if he stays away from the issue out of fear of being associated with leftist organizers.

Beyond questions of policy, there’s one more balancing act that Mr. Biden will have to master: assuaging concerns about his age (he’s 77) and mental acuity, without mentioning it explicitly. In an attack ad released this week, Mr. Trump’s campaign portrayed Mr. Biden as having lost his step in recent years, comparing a range of clips from 2015 and 2016 in which he speaks clearly and forcefully to videos from this year showing Mr. Biden fumbling for words.

There’s probably only one way Mr. Biden will be able to quiet those fears tonight: with a steady, smooth delivery — no matter what the substance is.