From a New York Times story by David Leonhardt headlined “Dueling Weaknesses”:

In 2016, when The New York Times’s pollsters asked Americans whether they planned to vote for Hillary Clinton or Donald Trump, more than 10 percent said they would not support either one. They said that they would instead vote for a third-party candidate or not vote at all.

Four years later, the situation was different. Joe Biden was a more popular nominee than Clinton had been, while some of Trump’s skeptics had come around to supporting him. Less than 5 percent of voters told pollsters that they didn’t plan to vote for either major party nominee.