Some of the 25 pivot counties that Mr. Biden did flip back reflect gains he made in suburbs across the country — the key to his election — or are in states like Pennsylvania, where his campaign poured in resources for advertising and voter outreach. Unlike in 2016, when complacency, lack of enthusiasm for the Democratic nominee and third-party candidates all cut into votes for Mrs. Clinton, this year there was a surge of Democratic turnout in the state, which Mr. Biden won by 7.1 points, compared with Mrs. Clinton’s 1.5-point margin in 2016. In Clay County, one of the four pivot counties in Minnesota that Mr. Biden wrangled back from Mr. Trump, Mr. Biden won 3,500 more votes than Mrs. Clinton did in 2016. 