Donald Trump’s presidential campaign waffled again on the details of its economic policy, with a top adviser suggesting a Trump administration would be open to reductions in Medicare and Social Security spending if the campaign’s tax cuts don’t achieve extraordinary budget surpluses.

To date, Mr. Trump has stood by his promise to leave untouched the entitlement benefits that older Americans receive, while proposing an unprecedentedly large tax cut. Outside analysts have concluded that combination would worsen the country’s...