Less than five months before the election, Mr. Trump has yet to publicly lay out a detailed vision for the next four years, instead touting what he marks as first-term achievements and arguing that his presumptive Democratic opponent, former Vice President Joe Biden, will undo many of the policies he has already put in place.

When The Wall Street Journal asked him last week to identify one new initiative he would undertake in his second term, Mr. Trump pointed to actions taken over the past three years and said he would focus on rebuilding the economy without naming specific policy plans…

Mr. Trump once hoped that the booming economy would ease his path to a second term, allowing him to make the argument that voters should expect more of the same if he is re-elected. Now, Mr. Trump may need to make the case that things will change if he wins in November.

“A campaign centered on more of the same—it’s not just out of step, it’s fatal,” said Republican strategist Kevin Madden, who worked for George W. Bush and Mitt Romney’s presidential campaigns.