With eight days to Nov. 5, President Biden on Monday stood in line with more than a hundred early voters in Delaware and cast his ballot for Vice President Kamala Harris.

It was an understated moment for a president who just three months ago thought he would be voting for himself. Asked whether it was bittersweet, Mr. Biden responded, “No, this is just sweet.”

Outside the polling location in New Castle, Del., in a deeply Democratic district near his hometown, Wilmington, the president greeted first-time voters, took photos and pushed a voter’s wheelchair as they waited their turns.

More than 20 million Americans have already made their pick during early in-person voting in a tight presidential contest. Mr. Biden cast his ballot for Ms. Harris while she was being whisked away to the Midwest for a campaign rally.

Mr. Biden showed nothing but confidence as he promoted the administration’s achievements in Pittsburgh this weekend, rallying support for Ms. Harris in a key battleground state.

“We’re gonna win Pennsylvania!” he said, pumping his first in the air before a dozen phone bank volunteers for the Allegheny County Labor Council on Saturday. Mr. Biden appeared with Senator John Fetterman, Democrat of Pennsylvania, and handed out pizza from a local restaurant to thank the volunteers.

Asked on Monday whether he wished more candidates wanted him to hit the trail with them, he suggested he was too busy with presidential duties.

“Well, I’ve done a lot of surrogacy,” Mr. Biden said. “But the fact of the matter is I’ve always had to be president at the same time.”

Four years ago, Mr. Biden had his wife, Jill Biden, at his side when he voted for himself. The first lady was not in Delaware on Monday to vote alongside her husband. Her schedule said she was attending back-to-back political events in Michigan and Wisconsin, campaigning for her husband’s successor in the Democratic Party.