Democrats say they have a simple plan for discrediting Robert F. Kennedy Jr. now that he has dropped out of the race and endorsed former President Donald J. Trump: calling him “weird.”

It’s a playground-style strategy that the Harris campaign has already deployed against Mr. Trump and his running mate, Senator JD Vance of Ohio, and the put-downs have proved popular with the Democratic base.

The party doesn’t have to look far for ammunition. Mr. Kennedy has recently made headlines for decapitating a dead whale with a chain saw and strapping its head to the family minivan, as well as dumping a deceased bear cub in Central Park. Now the Democrats want to attach his baggage to Mr. Trump.

On Friday, the Democratic National Committee unveiled billboards near the rally in Arizona where Mr. Kennedy, whose support once reached as high as 20 percent in some polls, endorsed the former president. The billboards showed an image of Mr. Trump flanked by Mr. Kennedy and Mr. Vance, with the text “Weird As Hell” in large type.

Democrats and their allies also plan to try to make Mr. Trump own some of Mr. Kennedy’s fringe positions and conspiratorial views, including his false statements about the dangers of vaccines and his brief call for a national abortion ban.

“A weirdo campaign just got weirder,” said Matt Bennett, a co-founder of Third Way, a Democratic think tank that has led efforts to stop independent and third-party candidates from siphoning votes from Democrats. “This campaign of freaks is not going to do Republicans any favors.”