Yet as Mr. Trump barrels ahead with crowded, risky rallies, some Democrats in battleground states are growing increasingly anxious about the trade-offs Mr. Biden has made. With some polls tightening since the beginning of the summer, they are warning him that virtual events may not be enough to excite voters, and urging him to intensify in-person outreach.

The concern among these Democrats is whether, in closely fought states that may be won on the margins, the Biden campaign is engaging every possible voter with an affirmative case for his candidacy, when the other side simply has more traditional tactics they are willing to use…

Compounding the challenge is an on-the-ground operation that was weak during the primary season and was slow to scale up in the general election. Strapped for cash after the primaries and uncertain about how to campaign amid a national lockdown, the Biden team initially refrained from greatly expanding its staff. It entered the summer without state directors in critical battlegrounds like Michigan, Florida and Pennsylvania, and efforts to establish local operations stretched deep into the summer.