Mr. Biden leads Mr. Trump 46 percent to 43 percent among likely voters in Iowa, with 7 percent saying they were undecided or refusing to name a preference, according to the survey. Senator Joni Ernst, a Republican whose re-election race could help determine control of the Senate, is capturing 45 percent support while Theresa Greenfield, her Democratic opponent, has 44 percent.

Mr. Biden, the former vice president, is being propelled by women, younger voters and white voters with college degrees, the same demographics lifting him across the country. Yet he is also running stronger in Iowa among seniors and working-class white voters than he is in other similarly Republican-leaning states.

Mr. Biden is leading among voters 65 and older, 49 percent to 42 percent, and he is trailing Mr. Trump among white voters without college degrees by only seven points, 48 percent to 41 percent.