Democratic presidential nominee Joe Biden has yet to map out his plans should he win office and Thursday night's speech at the Democratic National Convention might be his last best chance to do so, pollster Frank Luntz told “Special Report."

“What the Democrats have done so far is attack Donald Trump ... and they’ve talked about Joe Biden’s character,” Luntz told host Bret Baier Thursday. “What they have not done for the first three nights, and what Joe Biden has to do tonight, is talk about the issues, the substance. What changes is he going to bring? What policies is he going to promote?”

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Luntz said it’s also important for Biden to clarify whether he identifies as a centrist or a socialist, as Republicans often like to speculate. If Biden neglects to make that point, he said, the DNC will have been a failure.

“We do not know where Joe Biden stands,” he said. “Tonight is the best opportunity -- unedited, unfiltered – for him to explain to the American people where he really stands. And if he doesn’t do that, then this convention would not have been so successful.”

In contrast to the DNC’s gloomy overall narrative, Luntz predicted Biden would give an optimistic speech, saying that is "the kind of person that he is.”

“He is one of the more positive, one of the more inspirational candidates on the Democratic side,” he said. “And while it’s not beneath him to criticize and to attack, I think that’s what the other speakers, particularly Barack Obama, did over the last three nights.”

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“In the end," Luntz added, "voters know Donald Trump but they still don’t know Joe Biden specifically."