Robert Mueller has said that the report is his testimony.   The following imagines what Republicans might ask or say during Mr. Mueller’s testimony.   They probably wont.

Mueller: One, the Special Prosecutor’s team chose not to make a traditional charge/decline-to-charge decision.  The DoJ’s Office of Legal Council has ruled that a sitting president cannot be charged and the team accepted the reasoning behind this ruling. (p. 194.)

Republicans: However, you didn’t have to charge President Trump.  You could just have found that he did commit obstruction of justice, then leave it to Congress to follow through.  Impeachment is a constitutional process.  Why didn’t you find this conclusion?

Mueller: Two, the team investigated the facts in order to document occasions where other people had committed obstruction of justice[1] and to document cases where the President may have obstructed justice in order for him to be prosecuted after he leaves office.  (pp. 194-195.)

Mueller: Three, the team chose NOT to apply the common legal standard to the evidence that might have led to a decision that the President had committed a crime.  (p. 195.)

Republicans:

1) Why not?  Such a finding would lead to impeachment by the House.  See above.

2.) Or was that because he had not committed a crime?

Mueller: The Federal Government is a sieve, so news of a secret finding would leak.  This would cast a shadow over the President’s ability to lead.  (p. 195.)

Republicans: So has the Mueller Report cast a shadow over the President’s ability to lead?

Mueller: Four, the team can’t tell if President Trump obstructed justice or did not obstruct justice. (p. 195.)

Republicans (incautiously): Why is that?

Overarching factual issues.  (pp. 201-202.)

Mueller: It could not be a typical obstruction case because it concerned the President.

Mueller: First, some of his actions were “facially lawful,” but he also had official powers that could influence other people’s conduct. (p. 201.)

Mueller: Second, obstruction usually is intended to cover-up another crime, but the team did not establish that the President had committed any crime.  So the team had consider whether other motives inspired his actions.

Republicans: like punching back against what he believed to be an un-fair investigation?

Mueller: Third, the President often acted in full public view, rather than in secret.  Still, this might have been meant to influence witnesses.

[1] Such people can be prosecuted immediately.