I am not worried about my legacy. What I am worried about is the country,” Mr. Biden said. “Four years of Donald Trump will be very hard to overcome, but we can. Eight years of Donald Trump will fundamentally change the nature of who we are as a country. And it’ll take a generation, a generation or more for us to get back on track.”

This poll asked about four potential Democratic challengers to Trump in the 2020 presidential election, as shown in Table 12.

Former Vice President Joe Biden is favored by 50 percent and Trump by 44 percent, while 3 percent say they would not support either candidate and 3 percent say they don’t know. In August, Biden received 51 percent and Trump 42 percent.

Sen. Bernie Sanders receives 48 percent and Trump 46 percent, with 4 percent supporting neither and 2 percent who don’t know. In August, Sanders received 48 percent and Trump 44 percent.

Sen. Elizabeth Warren is the choice of 47 percent and Trump gets 46 percent, with 4 percent supporting neither and 2 percent saying they don’t know. In August, Warren received 45 percent and Trump 45 percent.

This is the first time the Marquette Law School Poll has matched Mayor Pete Buttigieg against Trump. Buttigieg is supported by 43 percent to Trump’s 45 percent, while 5 percent support neither and 7 percent say they don’t know.