On the campaign trail in late summer of 2024, Donald Trump stood at his Bedminster, N.J., club flanked by tables of everyday goods and small placards describing how much those items had risen in price during Joe Biden's presidency. Correctly assessing that this issue would be consequential in the race for the White House, Mr. Trump's campaign had smartly positioned him to highlight it. And it rang true with voters who weren't buying into the overly rosy messaging about the economy that was coming from the Biden administration. Now it is Mr. Trump who is claiming that voters should not believe their own wallets, calling the issue of affordability a "con job" by Democrats. The same trap that caught Mr. Biden (and subsequently his vice president, Kamala Harris) has now ensnared the president, and he has the job approval figures to match. Those numbers have fallen steadily this year and last month fell under 40 percent across multiple polls, rivaling the low-water mark of Mr. Trump's fi…