Robert Mueller's testimony Wednesday on Capitol Hill is sure to capture the nation. Mueller has said very little publicly about his two-year assignment investigating President Donald Trump and how Russia worked to benefit his 2016 presidential campaign. Now, he's in the klieg lights.

Still, Mueller is a reluctant witness, appearing not because he wants to—he said in May his 448-page report “speaks for itself”—but because House Democrats want to press him about how he arrived at certain conclusions, and what more he might want to say about Trump and his campaign's ties to Russia.

Trump has declared Mueller's report a “total exoneration,” and Democrats almost certainly will try to counter that assertion. Mueller didn't say whether he thinks Trump tried to obstruct the investigation, but the report also explicitly did not exonerate the president. Whether and how Mueller veers from his report—the expectation is that he will not—is consuming the echo chamber. “The report is my testimony,” Mueller said in May. “I would not provide information beyond that which is already public in any appearance before Congress.”