Julian Zelizer is a professor of history and public affairs at Princeton University, and author, with Kevin Kruse, of the new book "Fault Lines: A History of the United States Since 1974." Follow him on Twitter at @julianzelizer. The opinions expressed in this commentary are his own. View more opinion at CNN.

(CNN) Is Donald Trump another Jimmy Carter -- or George H.W. Bush? Democrats are hoping that Trump will join the short list of modern-era presidents who lost their bids for reelection. But the new Washington Post/ABC News poll should make them think twice.

Though a majority of Americans see President Trump as "unpresidential," his approval ratings have risen to 47% among registered voters and 44% among Americans of voting age. He is receiving credit from 51% of those polled for the strong economy. Regardless of who holds office, these are not the kinds of numbers that an opposition party likes to see.

Given that Speaker Nancy Pelosi has pretty much ruled out impeachment, almost every Democrat is hoping that the era of Trump comes to an end via the voting booth in November 2020.

Only then can the nation test Joe Biden's proposition that politics will resume to normal once someone else is in the Oval Office. Just as important, a Democratic president would have the opportunity to protect the policies and institutions which have come under threat in the past three years.

Democrats are thinking of Jimmy Carter and George H.W. Bush, the two presidents since Herbert Hoover who ran for reelection and failed. Ronald Reagan defeated President Carter in 1980 and Bill Clinton beat President Bush in 1992. Other than those two candidacies (Harry Truman and Lyndon Johnson both decided they would not run for reelection, Gerald Ford was never elected, and John Kennedy was assassinated), incumbent presidents have done well at the ballot box.

Read More