Democratic presidential candidate Pete Buttigieg claimed last month that “everything that we have proposed has been paid for, and we have proposed no tax increase on the middle class.” The South Bend, Ind., mayor is incorrect on both counts: He hasn’t said how he’d pay for all his proposed spending. He has endorsed one explicit tax increase on the middle class, and his recent retirement plan provides an outline for another. Add it up, and middle-class workers could face a trillion dollars in new taxes. To support family caregivers, Mr. Buttigieg’s retirement plan restated his prior commitment to enact “an...