WEST PALM BEACH, Fla.—President Trump’s advisers on Sunday began crafting plans to use special counsel Robert Mueller’s findings as a line of attack against Democrats ahead of the 2020 election, according to a person familiar with the matter, seeking to turn an investigation the White House long viewed as a liability into a political advantage.

Mr. Mueller didn’t find that the Trump campaign colluded with Russia’s efforts to interfere in the 2016 presidential election. He left unresolved questions about whether the president…