In 2004, Kerry was badly hurt by negative ads that raised questions about his decorated military record during the Vietnam War. While many of Kerry’s crewmates in the Navy defended his record and argued the so-called “swift boat” attacks were not credible, they hurt the Democrat with voters in the first presidential election after the September 11 attacks.

While the attacks were run by an outside group and not the Bush campaign, the effect was to focus an election centered on national security on Kerry’s readiness to lead rather than Bush’s White House record.

Independent experts note that the sheer uniqueness of Trump as a candidate makes it more plausible to cast the election as a referendum on the challenger than it might otherwise be.

“Usually, it is true, when you have a two-term president leaving, the election becomes about the retiring president and the effort is by the opposition to connect them together,” said Julian Zelizer, a Princeton University professor of history and public affairs. “This is how Obama attacked McCain in 2008, and Bush attacked Gore in 2000.

“But this year is different,” Zelizer added. “Given how outspoken and erratic Trump is, and given how well known he is as a public commodity, the dynamics have changed.”