It is very common for U.S. secondary schools and universities to grade students on a letter scale of A, B, C, D, and F. A is best, D is worst. It is also common to assign intermediate grades by adding + and - signs. Thus, C+ is better than C but worse than B-. Because this system is so familiar in the U.S., it is often used in other areas of life. Some political organizations, both liberal and conservative, like to give grades to senators. The grades express how often the senator voted "correctly." A liberal senator who almost always voted the liberal way might get a grade of "A+" from a liberal organization. The same senator might get a grade of "D-" from a conservative organization.

D-minus is a score, on a scale of A to E, with F being a failure, as in a school. A score of A or A-plus is the best. A D-minus score is relatively poor. The writer is using school or exam imagery to assess or comment on a politician's voting record. The question the intelligent reader should ask is, "What is meant by 'poor'?" Is it because the politician has been absent from voting (laziness; absenteeism)? Or is it because the writer does not like the way the politician has voted?"