What's a "Strong 'D'"?

From Politico:

"We just hoped the political diversity [in President-Elect Obama's cabinet] would have been stronger," said Tim Carpenter, executive director of Progressive Democrats of America. "We see a lot of recycled Clinton folks and he gets a strong 'D' on the policy side. We hope he will hustle them to be more progressive."

So is a strong "D" something better than an ordinary or weak D, like a D+ (a grade that often quite perplexed me in the message that it sent), so that Mr. Carpenter is trying to soften the blow of the D? Is it something that the speaker strongly feels is a D? Is it something that is strongly negative compared to a normal D, so it's a D-? I'm pretty sure it's not a strong "D" in the sense of Sen. Barack Obama (strong D-IL). Inquiring minds want to know.