In the final weeks of the 2020 election, President Donald J. Trump’s campaign surveyed likely voters in swing states about what political messages stuck with them.

Joseph R. Biden Jr.’s message, these voters said, centered on how Mr. Trump mishandled the coronavirus pandemic and was unfit for office. But for Mr. Trump, those surveyed echoed more than a dozen different messages, including his false claims about the virus, his third Supreme Court nomination and complaints that he deserved a Nobel Peace Prize. Just 3 percent of the voters recalled something specific Mr. Trump had said about Mr. Biden.

Now, some Trump advisers and allies say privately they are concerned that the dynamic may be repeating itself four years later. They worry that Mr. Trump’s impetuousness and scattershot style on the campaign trail needlessly risk victory in battleground states where the margin for error is increasingly narrow.

At a time when his opponent, Vice President Kamala Harris, has stepped up her attacks on him as “unstable,” Mr. Trump has struggled to publicly hone his message by veering off script and ramping up personal attacks on Ms. Harris that allies have urged him to rein in.

“When he’s good, he’s great, and when he’s off message, he’s not so great,” said David Urban, a Trump adviser. “I don’t think anyone is really changing their mind at this point, but when he distracts from his biggest, broadest messaging, it’s counterproductive because the Harris campaign uses it to turn out their voters.”

During a speech on Saturday in California, he described mail-in ballots as “so corrupt,” reviving one of his false attacks on the 2020 election results, and did a play-by-play of his internal thoughts when he watched SpaceX, Elon Musk’s spaceflight company, fly a rocket back onto its launch site.